ox: Introduce conditional/generated preamble

* lisp/ox.el (org-export-detect-features, org-export-expand-features,
org-export-generate-features-preamble): New functions for detecting
features and generating content based on them.
(org-export-conditional-features): Customisation for feature detection.
(org-export-as): Add detected to features to info in the slot :features.
(org-export-update-features): Add a convenience function for users to
edit the feature condition/implementation lists.
(org-export--annotate-info, org-export-detect-features,
org-export-define-derived-backend, org-export-define-backend,
org-export-conditional-features): Refactor backend feature
conditions/implementations into a struct field.  This allows for parent
inheritance to be properly managed, and leads into future work making
features more widely used in the export process.
(org-export-expand-features, org-export-resolve-feature-implementations,
org-export-generate-features-preamble,
org-export-expand-feature-snippets): The main functions for working with
export features.
(org-export-process-features, org-export-update-features): Introduce
`org-export-process-features' to simplify the application of features to
INFO.

lisp/org.el

@@ -3452,15 +3452,6 @@ header, or they will be appended."
     ;; inputenc and fontenc are for pdflatex only
     ("AUTO" "inputenc"  t ("pdflatex"))
     ("T1"   "fontenc"   t ("pdflatex"))
-    (""     "graphicx"  t)
-    (""     "longtable" nil)
-    (""     "wrapfig"   nil)
-    (""     "rotating"  nil)
-    ("normalem" "ulem"  t)
-    ;; amsmath and amssymb after inputenc/fontenc for pdflatex
-    (""     "amsmath"   t ("pdflatex"))
-    (""     "amssymb"   t ("pdflatex"))
-    (""     "capt-of"   nil)
     (""     "hyperref"  nil))
   "Alist of default packages to be inserted in the header.
 
@@ -3470,18 +3461,12 @@ incompatibility with another package you are using.
 The packages in this list are needed by one part or another of
 Org mode to function properly:
 
+- amsmath: for subscript and superscript and math environments
+  (automatically added when needed with pdflatex).
 - fontspec: for font and character selection in lualatex and xetex
 - inputenc, fontenc:  for basic font and character selection
-  in pdflatex
-- graphicx: for including images
-- longtable: For multipage tables
 - wrapfig: for figure placement
 - rotating: for sideways figures and tables
-- ulem: for underline and strike-through
-- amsmath: for subscript and superscript and math environments
-- amssymb: for various symbols used for interpreting the entities
-  in `org-entities'.  You can skip some of this package if you don't
-  use any of the symbols.
 - capt-of: for captions outside of floats
 - hyperref: for cross references
 

lisp/ox-beamer.el

@@ -847,9 +847,7 @@ holding export options."
      ;; Timestamp.
      (and (plist-get info :time-stamp-file)
 	  (format-time-string "%% Created %Y-%m-%d %a %H:%M\n"))
-     ;; LaTeX compiler
-     (org-latex--insert-compiler info)
-     ;; Document class and packages.
+     ;; Document class, packages, and some configuration.
      (org-latex-make-preamble info)
      ;; Define the alternative frame environment.
      (unless (equal "frame" org-beamer-frame-environment)
@@ -902,12 +900,6 @@ holding export options."
      (let ((template (plist-get info :latex-hyperref-template)))
        (and (stringp template)
 	    (format-spec template (org-latex--format-spec info))))
-     ;; engrave-faces-latex preamble
-     (when (and (eq (plist-get info :latex-src-block-backend) 'engraved)
-                (org-element-map (plist-get info :parse-tree)
-                    '(src-block inline-src-block) #'identity
-                    info t))
-       (org-latex-generate-engraved-preamble info))
      ;; Document start.
      "\\begin{document}\n\n"
      ;; Title command.

lisp/ox-latex.el

@@ -170,7 +170,83 @@
     (:latex-toc-command nil nil org-latex-toc-command)
     (:latex-compiler "LATEX_COMPILER" nil org-latex-compiler)
     ;; Redefine regular options.
-    (:date "DATE" nil "\\today" parse)))
+    (:date "DATE" nil "\\today" parse))
+  :feature-conditions-alist
+  `((t !announce-start !announce-end
+     !guess-pollyglossia !guess-babel !guess-inputenc)
+    (,(lambda (info)
+        ;; Since amsmath is added unconditionally when using
+        ;; xelatex/lualatex (see `org-latex-default-packages-alist'),
+        ;; and amssymb is not needed, we need not bother when using
+        ;; thoese compilers.
+        (and (not (member (plist-get info :latex-compiler) '("xelatex" "lualatex")))
+             (org-element-map (plist-get info :parse-tree)
+                 '(latex-fragment latex-environment) #'identity info t)))
+     maths)
+    (,(lambda (info)
+        (org-element-map (plist-get info :parse-tree)
+            'underline #'identity info t))
+     underline)
+    ("\\\\uu?line\\|\\\\uwave\\|\\\\sout\\|\\\\xout\\|\\\\dashuline\\|\\dotuline\\|\\markoverwith"
+     underline)
+    (,(lambda (info)
+        (org-element-map (plist-get info :parse-tree)
+            'link
+          (lambda (link)
+            (and (member (org-element-property :type link)
+                         '("http" "https" "ftp" "file"))
+                 (file-name-extension (org-element-property :path link))
+                 (equal (downcase (file-name-extension
+                                   (org-element-property :path link)))
+                        "svg")))
+          info t))
+     svg)
+    (org-latex-tables-booktabs booktabs)
+    (,(lambda (info)
+        (equal (plist-get info :latex-default-table-environment)
+               "longtable"))
+     longtable)
+    ("^[ \t]*\\+attr_latex: .*:environment +longtable"
+     longtable)
+    (,(lambda (info)
+        (eq (plist-get info :latex-src-block-backend) 'engraved))
+     engraved-code)
+    ("^[ \t]*#\\+attr_latex: .*:float +wrap"
+     float-wrap)
+    ("^[ \t]*#\\+attr_latex: .*:float +sideways"
+     rotate)
+    ("^[ \t]*#\\+caption\\(?:\\[.*\\]\\)?:\\|\\\\caption{" caption))
+  :feature-implementations-alist
+  `((!announce-start
+     :snippet ,(lambda (info)
+                 (with-temp-buffer
+                   (setq-local left-margin 2)
+                   (insert (string-join
+                            (mapcar #'symbol-name
+                                    (plist-get info :features))
+                            ", ")
+                           ".")
+                   (fill-region-as-paragraph (point-min) (point-max))
+                   (goto-char (point-min))
+                   (insert "%% ox-latex features:\n% ")
+                   (while (search-forward "\n" nil t)
+                     (insert "%"))
+                   (buffer-string)))
+     :order -100)
+    (maths :snippet "\\usepackage{amsmath}\n\\usepackage{amssymb}" :order 0.2)
+    (underline :snippet "\\usepackage[normalem]{ulem}" :order 0.5)
+    (image :snippet "\\usepackage{graphicx}" :order 2)
+    (svg :snippet "\\usepackage[inkscapelatex=false]{svg}" :order 2 :when image)
+    (longtable :snippet "\\usepackage{longtable}" :when table :order 2)
+    (booktabs :snippet "\\usepackage{booktabs}" :when table :order 2)
+    (float-wrap :snippet "\\usepackage{wrapfig}" :order 2)
+    (rotate :snippet "\\usepackage{rotating}" :order 2)
+    (caption :snippet "\\usepackage{capt-of}")
+    (engraved-code :when code :snippet org-latex-generate-engraved-preamble)
+    (!guess-pollyglossia :snippet org-latex-guess-polyglossia-language)
+    (!guess-babel :snippet org-latex-guess-babel-language)
+    (!guess-inputenc :snippet org-latex-guess-inputenc)
+    (!announce-end :snippet "%% end ox-latex features\n" :order 100)))
 
 
 
@@ -1353,7 +1429,7 @@ default values of which are given by `org-latex-engraved-preamble' and
              t t
              engraved-preamble)))
     (concat
-     "\n% Setup for code blocks [1/2]\n\n"
+     "% Setup for code blocks [1/2]\n\n"
      engraved-preamble
      "\n\n% Setup for code blocks [2/2]: syntax highlighting colors\n\n"
      (if (require 'engrave-faces-latex nil t)
@@ -1627,29 +1703,29 @@ For non-floats, see `org-latex--wrap-label'."
 	      (org-trim label)
 	      (org-export-data main info))))))
 
-(defun org-latex-guess-inputenc (header)
+(defun org-latex-guess-inputenc (info)
   "Set the coding system in inputenc to what the buffer is.
 
-HEADER is the LaTeX header string.  This function only applies
-when specified inputenc option is \"AUTO\".
+INFO is the plist used as a communication channel.
+This function only applies when specified inputenc option is \"AUTO\".
 
 Return the new header, as a string."
-  (let* ((cs (or (ignore-errors
-		   (latexenc-coding-system-to-inputenc
-		    (or org-export-coding-system buffer-file-coding-system)))
-		 "utf8")))
-    (if (not cs) header
+  (let ((header (plist-get info :latex-full-header))
+        (cs (or (ignore-errors
+                  (latexenc-coding-system-to-inputenc
+                   (or org-export-coding-system buffer-file-coding-system)))
+                "utf8")))
+    (when (and cs (string-match "\\\\usepackage\\[\\(AUTO\\)\\]{inputenc}" header))
       ;; First translate if that is requested.
       (setq cs (or (cdr (assoc cs org-latex-inputenc-alist)) cs))
-      ;; Then find the \usepackage statement and replace the option.
-      (replace-regexp-in-string "\\\\usepackage\\[\\(AUTO\\)\\]{inputenc}"
-				cs header t nil 1))))
+      (plist-put info :latex-full-header
+                 (replace-match cs t t header 1))))
+  nil)
 
-(defun org-latex-guess-babel-language (header info)
+(defun org-latex-guess-babel-language (info)
   "Set Babel's language according to LANGUAGE keyword.
 
-HEADER is the LaTeX header string.  INFO is the plist used as
-a communication channel.
+INFO is the plist used as a communication channel.
 
 Insertion of guessed language only happens when Babel package has
 explicitly been loaded.  Then it is added to the rest of
@@ -1663,52 +1739,48 @@ already loaded.
 
 Return the new header."
   (let* ((language-code (plist-get info :language))
-	 (plist (cdr
-		 (assoc language-code org-latex-language-alist)))
-	 (language (plist-get plist :babel))
-	 (language-ini-only (plist-get plist :babel-ini-only))
+         (plist (cdr (assoc language-code org-latex-language-alist)))
+         (language (plist-get plist :babel))
+         (header (plist-get info :latex-full-header))
+         (language-ini-only (plist-get plist :babel-ini-only))
          (language-ini-alt (plist-get plist :babel-ini-alt))
-	 ;; If no language is set, or Babel package is not loaded, or
-	 ;; LANGUAGE keyword value is a language served by Babel
-	 ;; exclusively through ini files, return HEADER as-is.
-	 (header (if (or language-ini-only
-			 (not (stringp language-code))
-			 (not (string-match "\\\\usepackage\\[\\(.*\\)\\]{babel}" header)))
-		     header
-		   (let ((options (save-match-data
-				    (org-split-string (match-string 1 header) ",[ \t]*"))))
-		     ;; If LANGUAGE is already loaded, return header
-		     ;; without AUTO.  Otherwise, replace AUTO with language or
-		     ;; append language if AUTO is not present.  Languages that are
-		     ;; served in Babel exclusively through ini files are not added
-		     ;; to the babel argument, and must be loaded using
-		     ;; `\babelprovide'.
-		     (replace-match
-		      (mapconcat (lambda (option) (if (equal "AUTO" option) language option))
-				 (cond ((member language options) (delete "AUTO" options))
-				       ((member "AUTO" options) options)
-				       (t (append options (list language))))
-				 ", ")
-		      t nil header 1)))))
+         (babel-header-options
+          ;; If no language is set, or Babel package is not loaded, or
+          ;; LANGUAGE keyword value is a language served by Babel
+          ;; exclusively through ini files, return HEADER as-is.
+          (and (not language-ini-only)
+               (stringp language-code)
+               (string-match "\\\\usepackage\\[\\(.*\\)\\]{babel}" header)
+               (let ((options (save-match-data
+                                (org-split-string (match-string 1 header) ",[ \t]*"))))
+                 (cond ((member language options) (delete "AUTO" options))
+                       ((member "AUTO" options) options)
+                       (t (append options (list language))))))))
+    (when babel-header-options
+      ;; If AUTO is present in the header options, replace it with `language'.
+      (setq header
+            (replace-match
+             (mapconcat (lambda (option) (if (equal "AUTO" option) language option))
+                        babel-header-options
+                        ", ")
+             t nil header 1)))
     ;; If `\babelprovide[args]{AUTO}' is present, AUTO is
     ;; replaced by LANGUAGE.
-    (if (not (string-match "\\\\babelprovide\\[.*\\]{\\(.+\\)}" header))
-	header
-      (let ((prov (match-string 1 header)))
-	(if (equal "AUTO" prov)
-	    (replace-regexp-in-string (format
-				       "\\(\\\\babelprovide\\[.*\\]\\)\\({\\)%s}" prov)
-				      (format "\\1\\2%s}"
-					      (if language-ini-alt language-ini-alt
-                                                (or language language-ini-only)))
-				      header t)
-	  header)))))
+    (when (string-match "\\\\babelprovide\\[.*\\]{AUTO}" header)
+      (setq header
+            (replace-regexp-in-string
+             (format
+              "\\(\\\\babelprovide\\[.*\\]\\)\\({\\)%s}" babel-header-options)
+             (format "\\1\\2%s}" (if language-ini-alt language-ini-alt
+                                   (or language language-ini-only)))
+             header t)))
+    (plist-put info :latex-full-header header))
+  nil)
 
-(defun org-latex-guess-polyglossia-language (header info)
+(defun org-latex-guess-polyglossia-language (info)
   "Set the Polyglossia language according to the LANGUAGE keyword.
 
-HEADER is the LaTeX header string.  INFO is the plist used as
-a communication channel.
+INFO is the plist used as a communication channel.
 
 Insertion of guessed language only happens when the Polyglossia
 package has been explicitly loaded.
@@ -1719,48 +1791,50 @@ replaced with the language of the document or
 using \setdefaultlanguage and not as an option to the package.
 
 Return the new header."
-  (let* ((language (plist-get info :language)))
+  (let ((header (plist-get info :latex-full-header))
+        (language (plist-get info :language)))
     ;; If no language is set or Polyglossia is not loaded, return
     ;; HEADER as-is.
-    (if (or (not (stringp language))
-	    (not (string-match
-		  "\\\\usepackage\\(?:\\[\\([^]]+?\\)\\]\\){polyglossia}\n"
-		  header)))
-	header
+    (when (and (stringp language)
+               (string-match
+                "\\\\usepackage\\(?:\\[\\([^]]+?\\)\\]\\){polyglossia}\n"
+                header))
       (let* ((options (org-string-nw-p (match-string 1 header)))
-	     (languages (and options
-			     ;; Reverse as the last loaded language is
-			     ;; the main language.
-			     (nreverse
-			      (delete-dups
-			       (save-match-data
-				 (org-split-string
-				  (replace-regexp-in-string
-				   "AUTO" language options t)
-				  ",[ \t]*"))))))
-	     (main-language-set
-	      (string-match-p "\\\\setmainlanguage{.*?}" header)))
-	(replace-match
-	 (concat "\\usepackage{polyglossia}\n"
-		 (mapconcat
-		  (lambda (l)
-		    (let* ((plist (cdr
-				   (assoc language org-latex-language-alist)))
-			   (polyglossia-variant (plist-get plist :polyglossia-variant))
-			   (polyglossia-lang (plist-get plist :polyglossia))
-			   (l (if (equal l language)
-				  polyglossia-lang
-				l)))
-		      (format (if main-language-set (format "\\setotherlanguage{%s}\n" l)
-				(setq main-language-set t)
-				"\\setmainlanguage%s{%s}\n")
-			      (if polyglossia-variant
-				  (format "[variant=%s]" polyglossia-variant)
-				"")
-			      l)))
-		  languages
-		  ""))
-	 t t header 0)))))
+             (languages (and options
+                             ;; Reverse as the last loaded language is
+                             ;; the main language.
+                             (nreverse
+                              (delete-dups
+                               (save-match-data
+                                 (org-split-string
+                                  (replace-regexp-in-string
+                                   "AUTO" language options t)
+                                  ",[ \t]*"))))))
+             (main-language-set
+              (string-match-p "\\\\setmainlanguage{.*?}" header))
+             (polyglossia-modified-header
+              (replace-match
+               (concat "\\usepackage{polyglossia}\n"
+                       (mapconcat
+                        (lambda (l)
+                          (let* ((plist (cdr (assoc language org-latex-language-alist)))
+                                 (polyglossia-variant (plist-get plist :polyglossia-variant))
+                                 (polyglossia-lang (plist-get plist :polyglossia))
+                                 (l (if (equal l language)
+                                        polyglossia-lang
+                                      l)))
+                            (format (if main-language-set (format "\\setotherlanguage{%s}\n" l)
+                                      (setq main-language-set t)
+                                      "\\setmainlanguage%s{%s}\n")
+                                    (if polyglossia-variant
+                                        (format "[variant=%s]" polyglossia-variant)
+                                      "")
+                                    l)))
+                        languages
+                        ""))
+               t t header 0)))
+        (plist-put info :latex-full-header polyglossia-modified-header))))
+  nil)
 
 (defun org-latex--remove-packages (pkg-alist info)
   "Remove packages based on the current LaTeX compiler.
@@ -1965,32 +2039,50 @@ non-nil, only includes packages relevant to image generation, as
 specified in `org-latex-default-packages-alist' or
 `org-latex-packages-alist'."
   (let* ((class (plist-get info :latex-class))
-	 (class-template
-	  (or template
-	      (let* ((class-options (plist-get info :latex-class-options))
-		     (header (nth 1 (assoc class (plist-get info :latex-classes)))))
-		(and (stringp header)
-		     (if (not class-options) header
-		       (replace-regexp-in-string
-			"^[ \t]*\\\\documentclass\\(\\(\\[[^]]*\\]\\)?\\)"
-			class-options header t nil 1))))
-	      (user-error "Unknown LaTeX class `%s'" class))))
-    (org-latex-guess-polyglossia-language
-     (org-latex-guess-babel-language
-      (org-latex-guess-inputenc
-       (org-element-normalize-string
-	(org-splice-latex-header
-	 class-template
-	 (org-latex--remove-packages org-latex-default-packages-alist info)
-	 (org-latex--remove-packages org-latex-packages-alist info)
-	 snippet?
-	 (mapconcat #'org-element-normalize-string
-		    (list (plist-get info :latex-header)
-			  (and (not snippet?)
-			       (plist-get info :latex-header-extra)))
-		    ""))))
-      info)
-     info)))
+         (class-template
+          (or template
+              (let* ((class-options (plist-get info :latex-class-options))
+                     (header (nth 1 (assoc class (plist-get info :latex-classes)))))
+                (and (stringp header)
+                     (if (not class-options) header
+                       (replace-regexp-in-string
+                        "^[ \t]*\\\\documentclass\\(\\(\\[[^]]*\\]\\)?\\)"
+                        class-options header t nil 1))))
+              (user-error "Unknown LaTeX class `%s'" class)))
+         generated-preamble)
+    (plist-put info :latex-full-header
+               (org-element-normalize-string
+                (org-splice-latex-header
+                 class-template
+                 (org-latex--remove-packages org-latex-default-packages-alist info)
+                 (org-latex--remove-packages org-latex-packages-alist info)
+                 snippet?
+                 (mapconcat #'org-element-normalize-string
+                            (list (plist-get info :latex-header)
+                                  (and (not snippet?)
+                                       (plist-get info :latex-header-extra)))
+                            ""))))
+    (setq generated-preamble
+          (if snippet?
+              (progn
+                (org-latex-guess-inputenc info)
+                (org-latex-guess-babel-language info)
+                (org-latex-guess-polyglossia-language info)
+                "\n% Generated preamble omitted for snippets.")
+            (concat
+             "\n"
+             (string-join
+              (org-export-expand-feature-snippets info)
+              "\n\n")
+             "\n")))
+    (concat
+     ;; Time-stamp.
+     (and (plist-get info :time-stamp-file)
+          (format-time-string "%% Created %Y-%m-%d %a %H:%M\n"))
+     ;; LaTeX compiler.
+     (org-latex--insert-compiler info)
+     (plist-get info :latex-full-header)
+     generated-preamble)))
 
 (defun org-latex-template (contents info)
   "Return complete document string after LaTeX conversion.
@@ -1999,12 +2091,7 @@ holding export options."
   (let ((title (org-export-data (plist-get info :title) info))
 	(spec (org-latex--format-spec info)))
     (concat
-     ;; Timestamp.
-     (and (plist-get info :time-stamp-file)
-	  (format-time-string "%% Created %Y-%m-%d %a %H:%M\n"))
-     ;; LaTeX compiler.
-     (org-latex--insert-compiler info)
-     ;; Document class and packages.
+     ;; Timestamp, compiler statement, document class and packages.
      (org-latex-make-preamble info)
      ;; Possibly limit depth for headline numbering.
      (let ((sec-num (plist-get info :section-numbers)))
@@ -2041,12 +2128,6 @@ holding export options."
      (let ((template (plist-get info :latex-hyperref-template)))
        (and (stringp template)
             (format-spec template spec)))
-     ;; engrave-faces-latex preamble
-     (when (and (eq (plist-get info :latex-src-block-backend) 'engraved)
-                (org-element-map (plist-get info :parse-tree)
-                    '(src-block inline-src-block) #'identity
-                    info t))
-       (org-latex-generate-engraved-preamble info))
      ;; Document start.
      "\\begin{document}\n\n"
      ;; Title command.

lisp/ox.el

@@ -1060,7 +1060,7 @@ mode."
 
 (cl-defstruct (org-export-backend (:constructor org-export-create-backend)
 				  (:copier nil))
-  name parent transcoders options filters blocks menu)
+  name parent transcoders options filters blocks menu feature-conditions feature-implementations)
 
 ;;;###autoload
 (defun org-export-get-backend (name)
@@ -1166,6 +1166,62 @@ returns filters inherited from parent backends, if any."
 	(setq filters (append filters (org-export-backend-filters backend))))
       filters)))
 
+(defvar org-export-conditional-features)
+
+(defun org-export-get-all-feature-conditions (backend)
+  "Return full feature condition alist for BACKEND.
+
+BACKEND is an export back-end, as return by, e.g,,
+`org-export-create-backend'.  Return value is an alist where keys
+are feature conditions, and values are feature symbols.
+
+Unlike `org-export-backend-feature-conditions', this function
+also returns conditions inherited from parent back-ends, if any."
+  (when (symbolp backend) (setq backend (org-export-get-backend backend)))
+  (and backend
+       (let ((conditions (org-export-backend-feature-conditions backend))
+             parent)
+         (while (setq parent (org-export-backend-parent backend))
+           (setq backend (org-export-get-backend parent))
+           (dolist (condition (org-export-backend-feature-conditions backend))
+             (push condition conditions)))
+         (dolist (condition org-export-conditional-features)
+           (unless (assq (car condition) conditions)
+             (push condition conditions)))
+         conditions)))
+
+(defun org-export-get-all-feature-implementations (backend)
+  "Return full feature implementation alist for BACKEND.
+
+BACKEND is an export back-end, as return by, e.g,,
+`org-export-create-backend'.  Return value is an alist where keys
+are feature symbols, and values are an implementation
+specification plist.
+
+Unlike `org-export-backend-feature-implementations', this function
+also returns implementations inherited from parent back-ends, if any."
+  (when (symbolp backend) (setq backend (org-export-get-backend backend)))
+  (and backend
+       (let ((implementations (org-export-backend-feature-implementations backend))
+             parent)
+         (while (setq parent (org-export-backend-parent backend))
+           (setq backend (org-export-get-backend parent))
+           (dolist (implementation (org-export-backend-feature-implementations backend))
+             (unless (assq (car implementation) implementations)
+               (push implementation implementations))))
+         implementations)))
+
+(defun org-export-install-features (info)
+  "Install feature conditions and implementations in the communication channel.
+INFO is a plist containing the current communication channel.
+Return the updated communication channel."
+  (plist-put info :feature-conditions
+             (org-export-get-all-feature-conditions
+              (plist-get info :back-end)))
+  (plist-put info :feature-implementations
+             (org-export-get-all-feature-implementations
+              (plist-get info :back-end))))
+
 (defun org-export-define-backend (backend transcoders &rest body)
   "Define a new backend BACKEND.
 
@@ -1277,20 +1333,24 @@ keywords are understood:
     `org-export-options-alist' for more information about
     structure of the values."
   (declare (indent 1))
-  (let (filters menu-entry options)
+  (let (filters menu-entry options feature-conditions feature-implementations)
     (while (keywordp (car body))
       (let ((keyword (pop body)))
 	(pcase keyword
 	  (:filters-alist (setq filters (pop body)))
 	  (:menu-entry (setq menu-entry (pop body)))
 	  (:options-alist (setq options (pop body)))
+	  (:feature-conditions-alist (setq feature-conditions (pop body)))
+	  (:feature-implementations-alist (setq feature-implementations (pop body)))
 	  (_ (error "Unknown keyword: %s" keyword)))))
     (org-export-register-backend
      (org-export-create-backend :name backend
 				:transcoders transcoders
 				:options options
 				:filters filters
-				:menu menu-entry))))
+				:menu menu-entry
+                                :feature-conditions feature-conditions
+                                :feature-implementations feature-implementations))))
 
 (defun org-export-define-derived-backend (child parent &rest body)
   "Create a new backend as a variant of an existing one.
@@ -1337,7 +1397,7 @@ The backend could then be called with, for example:
 
   (org-export-to-buffer \\='my-latex \"*Test my-latex*\")"
   (declare (indent 2))
-  (let (filters menu-entry options transcoders)
+  (let (filters menu-entry options transcoders feature-conditions feature-implementations)
     (while (keywordp (car body))
       (let ((keyword (pop body)))
 	(pcase keyword
@@ -1345,6 +1405,8 @@ The backend could then be called with, for example:
 	  (:menu-entry (setq menu-entry (pop body)))
 	  (:options-alist (setq options (pop body)))
 	  (:translate-alist (setq transcoders (pop body)))
+	  (:feature-conditions-alist (setq feature-conditions (pop body)))
+	  (:feature-implementations-alist (setq feature-implementations (pop body)))
 	  (_ (error "Unknown keyword: %s" keyword)))))
     (org-export-register-backend
      (org-export-create-backend :name child
@@ -1352,7 +1414,9 @@ The backend could then be called with, for example:
 				:transcoders transcoders
 				:options options
 				:filters filters
-				:menu menu-entry))))
+				:menu menu-entry
+                                :feature-conditions feature-conditions
+                                :feature-implementations feature-implementations))))
 
 
 
@@ -2055,6 +2119,550 @@ keywords before output."
 	    (funcall (intern (format "org-element-%s-interpreter" type))
 		     blob contents))))
 
+
+;;; Conditional/Generated Features
+;;
+;; Many formats have some version of a preamble, whether it be HTML's
+;; <head>...</head> or the content before LaTeX's \begin{document}.
+;; Depending on the particular features in the Org document being
+;; exported, different setup snippets will be needed.  There's the
+;; "everything and the kitchen sink" approach of adding absolutely
+;; everything that might be needed, and the post-translation editing
+;; with filters approach, but neither really solve this problem nicely.
+;;
+;; The conditional/generated preamble defines mechanisms of detecting
+;; which "export features" are used in a document, handles
+;; interactions between features, and provides/generates content to
+;; support the features.
+;;
+;; Each export feature condition takes the form of a
+;; (CONDITION . FEATURES) cons cell (see `org-export-detect-features'),
+;; and each implementation takes the form of a (FEATURE . (:KEY VALUE ...))
+;; associated plist (see `org-export-resolve-feature-implementations'
+;; and `org-export-expand-feature-snippets').
+;;
+;; This functionality is applied during export as follows:
+;; 1. The export feature conditions and implementations are installed
+;;    into the INFO plist with `org-export-install-features'.
+;;    This simply applies `org-export-get-all-feature-conditions' and
+;;    `org-export-get-all-feature-implementations', which merges the
+;;    backend's conditions/implementations with all of it's parents and
+;;    finally the global condition list
+;;    `org-export-conditional-features'.
+;; 2. The "export features" used in a document are detected with
+;;    `org-export-detect-features'.
+;; 3. The interaction between different feature implementations is
+;;    resolved with `org-export-resolve-feature-implementations',
+;;    producing an ordered list of implementations to be actually used
+;;    in an export.
+;; 4. The feature implementation's snippets are transformed into strings
+;;    to be inserted with `org-export-expand-feature-snippets'.
+
+(defcustom org-export-conditional-features
+  `(("^[ \t]*#\\+print_bibliography:" bibliography)
+    (,(lambda (info)
+       (org-element-map (plist-get info :parse-tree)
+           'link
+         (lambda (link)
+           (and (member (org-element-property :type link)
+                        '("http" "https" "ftp" "file"))
+                (file-name-extension (org-element-property :path link))
+                (member (downcase (file-name-extension
+                                   (org-element-property :path link)))
+                        image-file-name-extensions)))
+         info t))
+     image)
+    (,(lambda (info)
+       (org-element-map (plist-get info :parse-tree)
+           'table #'identity info t))
+     table)
+    (,(lambda (info)
+       (org-element-map (plist-get info :parse-tree)
+           '(src-block inline-src-block) #'identity info t))
+     code))
+  "Org feature tests and associated feature flags.
+
+Alist where the car is a test for the presense of the feature,
+and the CDR is either a single feature symbol or a list of
+feature symbols.
+
+See `org-export-detect-features' for how this is processed."
+  :group 'org-export-general
+  :type '(alist :key-type
+                (choice (regexp :tag "Feature test regexp")
+                        (variable :tag "Feature variable")
+                        (function :tag "Feature test function"))
+                :value-type
+                (repeat symbol :tag "Feature symbols")))
+
+(defun org-export-detect-features (info)
+  "Detect features from `org-export-conditional-features' in INFO.
+
+More specifically, for each (CONDITION . FEATURES) cons cell of
+the :feature-conditions list in INFO, the CONDITION is evaluated
+in two phases.
+
+In phase one, CONDITION is transformed like so:
+- If a variable symbol, the value is fetched
+- If a function symbol, the function is called with INFO as the
+  sole argument
+- If a string, passed on unmodified
+
+In phase two, if the CONDITION result is a string, it is used as
+a case-sensitive regexp search in the buffer.  The regexp
+matching is taken as confirmation of the existance of FEATURES.
+Any other non-nil value indicates the existance of FEATURES.
+
+A list of all detected feature symbols is returned.
+
+This function should be run in the processed export Org buffer,
+after includes have been expanded and commented trees removed."
+  (delete-dups
+   (cl-loop
+    for (condition . features) in (plist-get info :feature-conditions)
+    for matcher =
+    (cond
+     ((stringp condition) condition)
+     ((functionp condition) (funcall condition info))
+     ((symbolp condition) (symbol-value condition))
+     (t (error "org-export: Feature condition %s (for %s) unable to be used"
+               condition features)))
+    for active-features =
+    (and (if (stringp matcher)
+             (save-excursion
+               (goto-char (point-min))
+               (re-search-forward matcher nil t))
+           matcher)
+         (copy-sequence features))
+    when active-features
+    nconc active-features)))
+
+(define-error 'org-missing-feature-dependency
+  "A feature was asked for, but is not availible")
+
+(define-error 'org-circular-feature-dependency
+  "There was a circular dependency between some features")
+
+(defun org-export-resolve-feature-implementations (info &optional features implementations)
+  "Resolve the IMPLEMENTATIONS of FEATURES, of INFO.
+
+FEATURES should be a list of all feature symbols to be resolved,
+and defaults to (plist-get info :features).  IMPLEMENTATIONS
+should be an alist of feature symbols and specification plists,
+and defaults to (plist-get info :feature-implementations).
+
+The following keys of the each implementation plist are recognised:
+- :snippet, which is either,
+  - A string, which should be included in the preamble verbatim.
+  - A variable, the value of which should be included in the preamble.
+  - A function, which is called with two arguments — the export info,
+    and the list of feature flags.  The returned value is included in
+    the preamble.
+- :requires, a feature or list of features this feature will enable.
+- :when, a feature or list of features which are required for this
+  feature to be active.
+- :prevents, a feature or list of features that should be masked.
+- :order, for when inclusion order matters.  Feature implementations
+  with a lower order appear first.  The default is 0.
+- :after, a feature or list of features that must be preceding.
+- :before, a feature or list of features that must be succeeding.
+
+This function processes :requires, :when, and :prevents in turn,
+sorting according by :order both before processing :requires and
+after processing :prevents.  The final implementation list is
+returned."
+  (let* ((explicit-features (or features (plist-get info :features)))
+         (implementations (or implementations
+                              (plist-get info :feature-implementations)))
+         (current-implementations
+          (sort (cl-loop for feat in explicit-features
+                         collect (assq feat implementations))
+                (lambda (a b)
+                  (< (or (plist-get (cdr a) :order) 0)
+                     (or (plist-get (cdr b) :order) 0)))))
+         ;; require-records serves to record /why/ a particular implementation
+         ;; is used.  It takes the form of an alist with feature symbols as the
+         ;; keys, and a list of features that ask for that feature as values.
+         ;; A t value is used to indicate the feature has been explicitly
+         ;; required.
+         (require-records
+          (cl-loop for feat in explicit-features
+                   collect (list feat t))))
+    ;; * Process ~:requires~
+    ;; Here we temporarily treat current-implementations as a queue of
+    ;; unproceesed implementations, and for each implemention move
+    ;; it to processed-implementations if not already present.
+    ;; :requires are processed by being added to the current-implementations
+    ;; stack as they are seen.  Along the way require-records is built for
+    ;; the sake of the subsequent :prevents processing.
+    (let ((impl-queue-last (last current-implementations))
+          processed-implementations impl)
+      (while current-implementations
+        (setq impl (pop current-implementations))
+        (unless (memq impl processed-implementations)
+          (push impl processed-implementations)
+          (dolist (req (org-ensure-list
+                        (plist-get (cdar processed-implementations) :requires)))
+            (unless (assq req processed-implementations)
+              (let ((required-impl (assq req implementations)))
+                (unless required-impl
+                  (signal 'org-missing-feature-dependency
+                          (format "The feature `%s' was asked for but could not be found"
+                                  req)))
+                (setq impl-queue-last
+                      (if current-implementations
+                          (setcdr impl-queue-last (list required-impl))
+                        (setq current-implementations (list required-impl))))
+                (push (car impl) (alist-get req require-records)))))))
+      (setq current-implementations
+            (nreverse (delq nil processed-implementations))))
+    ;; * Process ~:when~
+    ;; More specifically, remove features with unfulfilled :when conditions.
+    ;; To correctly resolve all the various :when conditions,
+    ;; do not make any assumptions about which features are active.
+    ;; Initially only consider non-:when implementations to be
+    ;; active, then run through the list of unconfirmed :when
+    ;; implementations and check their conditions against the list
+    ;; of confirmed features.  Continue doing this until no more
+    ;; features are confirmed.
+    (let ((processing t)
+          (confirmed-features
+           (cl-remove-if ; Count unimplemented features as present.
+            (lambda (feat) (assq feat current-implementations))
+            explicit-features))
+          conditional-implementations when)
+      ;; Sort all features by the presense of :when.
+      (dolist (impl current-implementations)
+        (if (plist-get (cdr impl) :when)
+            (push impl conditional-implementations)
+          (push (car impl) confirmed-features)))
+      (while processing
+        (setq processing nil)
+        ;; Check for implementations which have satisfied :when
+        ;; contions.
+        (dolist (impl conditional-implementations)
+          (setq when (plist-get (cdr impl) :when))
+          (when (cond
+                 ((symbolp when)
+                  (memq when confirmed-features))
+                 ((consp when)
+                  (not (cl-set-difference when confirmed-features))))
+            (push (car impl) confirmed-features)
+            (setq conditional-implementations
+                  (delq impl conditional-implementations)
+                  processing t))))
+      ;; Now all that remains is implementations with unsatisfiable
+      ;; :when conditions.
+      (dolist (impl conditional-implementations)
+        (setq current-implementations
+              (delq impl current-implementations))))
+    ;; * Process ~:prevents~
+    ;; Go through every implementation and for prevented features
+    ;; 1. Remove them from current-implementations
+    ;; 2. Go through require-records and remove them from the cdrs.
+    ;; By modifying require-records in this way, features that are
+    ;; only present due to a now-prevented feature will have a
+    ;; nil cdr.  We can then (recursively) check for these features
+    ;; with `rassq' and remove them.
+    ;; Since we used a queue rather than a stack when processing
+    ;; :requires, we know that second order requires (i.e. :requires
+    ;; of :requires) will come after after first order requires.
+    ;; This means that should a n-th order require be prevented by
+    ;; (n-1)-th order require, it will be removed before being
+    ;; processed, and hence handled correctly.
+    (let (feats-to-remove removed null-require)
+      (dolist (impl current-implementations)
+        (setq feats-to-remove (org-ensure-list (plist-get (cdr impl) :prevents)))
+        (while feats-to-remove
+          ;; Remove each of feats-to-remove.
+          (dolist (feat feats-to-remove)
+            (unless (memq feat removed)
+              (push feat removed)
+              (setq current-implementations
+                    (delq (assq feat current-implementations)
+                          current-implementations))
+              (when (assq feat require-records)
+                (setq require-records
+                      (delq (assq feat require-records) require-records)))))
+          (dolist (rec require-records)
+            (setcdr rec (cl-set-difference (cdr rec) feats-to-remove)))
+          ;; The features have now been removed.
+          (setq feats-to-remove nil)
+          ;; Look for orphan requires.
+          (when (setq null-require (rassq nil require-records))
+            (push (car null-require) feats-to-remove)))))
+    ;; Re-sort by ~:order~, to position reqirued features correctly.
+    (setq current-implementations
+          (sort current-implementations
+                (lambda (a b)
+                  (< (or (plist-get (cdr a) :order) 0)
+                     (or (plist-get (cdr b) :order) 0)))))
+    ;; * Processing ~:before~ and ~:after~
+    ;; To resolve dependency order, we will now perform a stable topological
+    ;; sort on any DAGs that exist within current-implementations.
+    (org-export--feature-implementation-toposort
+     current-implementations)))
+
+(defun org-export--feature-implementation-toposort (implementations)
+  "Perform a stable topological sort of IMPLEMENTATIONS.
+The sort is performed based on the :before and :after properties.
+
+See <https://en.wikipedia.org/wiki/Topological_sorting> for more information
+on what this entails."
+  (let ((feature-indicies
+         (cl-loop
+          for elt in implementations
+          and index from 0
+          collect (cons (car elt) index)))
+        resolved-implementations
+        adj-list node-stack)
+    ;; Build an adjacency list from :before and :after.
+    (dolist (impl implementations)
+      (push (list (car impl)) adj-list))
+    (dolist (impl implementations)
+      (let ((before (org-ensure-list (plist-get (cdr impl) :before)))
+            (after (org-ensure-list (plist-get (cdr impl) :after))))
+        (dolist (child before)
+          (push (car impl) (cdr (assq child adj-list))))
+        (when after
+          (setcdr (assq (car impl) adj-list)
+                  (nconc (cdr (assq (car impl) adj-list))
+                         after)))))
+    ;; Initialise the node stack with the first implementation.
+    (setq node-stack (list (car implementations))
+          ;; Make the order of adj-list match implementations.
+          adj-list
+          (mapcar
+           (lambda (entry)
+             (cons (car entry)
+                   ;; Sort edges according to feature order, to do
+                   ;; the DFS in order and make the result stable.
+                   (sort (cdr entry)
+                         (lambda (a b)
+                           (< (or (alist-get a feature-indicies)
+                                  most-positive-fixnum)
+                              (or (alist-get b feature-indicies)
+                                  most-positive-fixnum))))))
+           (nreverse adj-list)))
+    (while adj-list
+      (let ((deps (alist-get (caar node-stack) adj-list))
+            new-dep-found)
+        ;; Look for any unresolved dependencies.
+        (while (and deps (not new-dep-found))
+          (if (not (assq (car deps) adj-list))
+              (setq deps (cdr deps))
+            ;; Check the unresolved dependency is not part of a cycle.
+            (when (assq (car deps) node-stack)
+              (signal 'org-circular-feature-dependency
+                      (format "Found a cycle in the feature dependency graph: %S"
+                              (cons (car deps)
+                                    (nreverse (memq (car deps)
+                                                    (nreverse
+                                                     (mapcar #'car node-stack))))))))
+            ;; Push the unresolved dependency to the top of the stack.
+            (push (assq (car deps) implementations)
+                  node-stack)
+            (setq new-dep-found t)))
+        (unless new-dep-found
+          ;; The top item of the stack has no unresolved dependencies.
+          ;; Move it to the resolved list, and remove its entry from
+          ;; adj-list to both mark it as such and ensure that
+          ;; node-stack will not be incremented to it when/if the
+          ;; stack is emptied.
+          (push (car node-stack) resolved-implementations)
+          (setq adj-list
+                (delq (assq (caar node-stack) adj-list) adj-list)
+                node-stack
+                (or (cdr node-stack)
+                    (list (assq (caar adj-list) implementations)))))))
+    (nreverse resolved-implementations)))
+
+(defun org-export-expand-feature-snippets (info &rest feature-implementations)
+  "Expand each of the feature :snippet keys in FEATURE-IMPLEMENTATIONS.
+FEATURE-IMPLEMENTATIONS is expected to be a list of implementation
+plists, if not provided explicitly it is extracted from the
+:feature-implementations key of INFO.  Note that an explicitly
+provided nil FEATURE-IMPLEMENTATIONS is interpreted as no features.
+
+Each implementation plist's :snippet value is expanded in order, in
+the following manner:
+- nil values are ignored
+- functions are called with INFO, and must produce a string or nil
+- variable symbols use the value, which must be a string or nil
+- strings are included verbatim
+- all other values throw an `error'.
+
+\(fn INFO &optional FEATURE-IMPLEMENTATIONS)"
+  (let ((feat-impls
+         (cond
+          ((not feature-implementations)
+           (plist-get info :feature-implementations))
+          ((= (length feature-implementations) 1)
+           (car feature-implementations))
+          (t (signal 'wrong-number-of-arguments
+                     `(org-export-expand-feature-snippets
+                       ,(1+ (length feature-implementations)))))))
+        expanded-snippets snippet value)
+    (dolist (impl feat-impls)
+      (setq snippet (plist-get (cdr impl) :snippet)
+            value (cond
+                   ((null snippet) nil)
+                   ((functionp snippet) (funcall snippet info))
+                   ((symbolp snippet) (symbol-value snippet))
+                   ((stringp snippet) snippet)
+                   (t (error "org-export: The %s feature snippet %S is invalid (must be either nil, a function/variable symbol, or a string)"
+                             (car impl) snippet))))
+      (cond
+       ((stringp value)
+        (push value expanded-snippets))
+       (value ; Non-string value, could come from function or variable.
+        (error "org-export: The %s feature snippet %s must give nil or a string, but instead gave %S"
+               (car impl)
+               (cond
+                ((and (functionp snippet) (symbolp snippet))
+                 (format "function (`%s')" snippet))
+                ((functionp snippet) "anonymous function")
+                (t (format "variable (`%s')" snippet)))
+               value))))
+    (nreverse expanded-snippets)))
+
+(defun org-export-process-features (info)
+  "Install feature conditions/implementations in INFO, and resolve them.
+See `org-export-detect-features' and `org-export-resolve-feature-implementations' for
+more information on what this entails."
+  (org-export-install-features info)
+  (let* ((exp-features (org-export-detect-features info))
+         (resolved-implementations
+          (org-export-resolve-feature-implementations info exp-features)))
+    (plist-put info :feature-implementations resolved-implementations)
+    (plist-put info :features (mapcar #'car resolved-implementations))))
+
+;;;###autoload
+(defmacro org-export-update-features (backend &rest feature-property-value-lists)
+  "For BACKEND's export spec, set each FEATURE's :PROPERTY to VALUE.
+
+The behaviour of this macro is best behaved with an example.
+For instance, to add some preamble content from the variable
+\"my-org-beamer-metropolis-tweaks\" when using the metropolis theme
+with beamer export:
+
+  (org-export-update-features \\='beamer
+    (beamer-metropolis
+     :condition (string-match-p \"metropolis$\" (plist-get info :beamer-theme))
+     :snippet my-org-beamer-metropolis-tweaks
+     :order 3))
+
+The modifies the beamer backend, either creating or updating the
+\"beamer-metropolis\" feature.  The :condition property adds a
+condition which detects the feature, and all other properties are
+applied to the feature's implementation plist.  Setting
+:condition to t means the feature will always be enabled, and
+conversely setting :condition to nil means the feature will never
+be enabled.
+
+When setting the :condition and :snippet properties, any sexp is
+is implicitly converted to,
+  (lambda (info) SEXPR)
+
+Each (FEATURE . (:PROPERTY VALUE)) form that is processed is
+taken from the single &rest argument
+FEATURE-PROPERTY-VALUE-LISTS.
+
+\(fn BACKEND &rest (FEATURE . (:PROPERTY VALUE)...)...)"
+  (declare (indent 1))
+  (org-with-gensyms (backend-struct the-entry the-condition the-feat-impl cond-feat)
+    (let ((backend-expr
+           (if (and (eq (car-safe backend) 'quote)
+                    (symbolp (cadr backend))
+                    (not (cddr backend)))
+               `(org-export-get-backend ',(cadr backend))
+             `(if (symbolp ,backend)
+                  (org-export-get-backend ,backend)
+                backend)))
+          (backend-impls
+           (list 'aref backend-struct
+                 (cl-struct-slot-offset 'org-export-backend 'feature-implementations)))
+          (backend-conds
+           (list 'aref backend-struct
+                 (cl-struct-slot-offset 'org-export-backend 'feature-conditions)))
+          body condition-set-p implementation-set-p)
+      (dolist (feature-property-value-set feature-property-value-lists)
+        (when (eq (car feature-property-value-set) 'quote)
+          (pop feature-property-value-set))
+        (let ((features (car feature-property-value-set))
+              (property-value-pairs (cdr feature-property-value-set))
+              let-body property value)
+          (while property-value-pairs
+            (setq property (pop property-value-pairs)
+                  value (pop property-value-pairs))
+            (cond
+             ((consp value)
+              (unless (memq (car value) '(function quote))
+                (if (and (memq property '(:condition :snippet))
+                         (not (functionp value)))
+                    (setq value `(lambda (info) ,value))
+                  (setq value (list 'quote value)))))
+             ((memq value '(nil t))) ; Leave unmodified.
+             ((symbolp value)
+              (setq value (list 'quote value))))
+            (if (eq property :condition)
+                (progn
+                  (unless condition-set-p
+                    (setq condition-set-p t))
+                  (push
+                   (if value
+                       (let ((the-features (org-ensure-list features)))
+                         `(let* ((,the-condition ,value)
+                                 (,the-entry (assoc ,the-condition ,backend-conds)))
+                            (if ,the-entry
+                                (setcdr ,the-entry
+                                        (append ',the-features (cdr ,the-entry)))
+                              (push (cons ,the-condition ',the-features)
+                                    ,backend-conds))))
+                     (let ((single-feature
+                            (if (consp features)
+                                (intern (string-join (mapcar #'symbol-name features)
+                                                     "-and-"))
+                              features)))
+                       `(dolist (,cond-feat ,backend-conds)
+                          (cond
+                           ((equal (cdr ,cond-feat) (list ',single-feature))
+                            (setf ,backend-conds (delq ,cond-feat ,backend-conds)))
+                           ((memq ',single-feature (cdr ,cond-feat))
+                            (setcdr ,cond-feat
+                                    (delq ',single-feature (cdr ,cond-feat))))))))
+                   body))
+              (unless implementation-set-p
+                (setq implementation-set-p t))
+              (push
+               (if let-body
+                   `(plist-put (cdr ,the-feat-impl) ,property ,value)
+                 `(setcdr ,the-feat-impl
+                   (plist-put (cdr ,the-feat-impl) ,property ,value)))
+               let-body)))
+          (when let-body
+            (let ((the-feature
+                   (if (consp features)
+                       (intern (string-join (mapcar #'symbol-name features)
+                                            "-and-"))
+                     features)))
+              (when (consp features)
+                (push
+                 `(plist-put (cdr ,the-feat-impl) :when ',features)
+                 let-body))
+              (push
+               `(let ((,the-feat-impl
+                       (or (assoc ',the-feature ,backend-impls)
+                           (car (push (list ',the-feature ,property nil)
+                                      ,backend-impls)))))
+                  ,@(nreverse let-body))
+               body)))))
+      `(let ((,backend-struct ,backend-expr))
+         ,@(and (not (org-export-backend-p backend-expr))
+                `((unless (org-export-backend-p ,backend-struct)
+                    (error "`%s' is not a loaded export backend" ,backend))))
+         ,@(nreverse body)
+         nil))))
 
 
 ;;; The Filter System
@@ -3129,6 +3737,8 @@ still inferior to file-local settings."
     (when (plist-get info :with-cite-processors)
       (org-cite-process-citations info)
       (org-cite-process-bibliography info))
+    ;; Install all the feature conditions and implementations.
+    (org-export-process-features info)
     info))
 
 ;;;###autoload