* lisp/org-element.el (org-element-subscript-parser): Fix edge case

Fix when both subscript and underline object match in
`org-element--object-lex'.
*
testing/lisp/test-org-element.el (test-org-element/underline-parser):
Add new test.

Reported-by: Tom Alexander <tom@fizz.buzz>
Link: https://orgmode.org/list/dad964f5-c764-4dd5-9829-ca38e3fbeb0d@app.fastmail.com

lisp/org-element.el

@@ -4087,24 +4087,25 @@ nil.
 
 Assume point is at the underscore."
   (save-excursion
-    (unless (bolp) (backward-char))
-    (when (looking-at org-match-substring-regexp)
-      (let ((bracketsp (if (match-beginning 4) t nil))
-	    (begin (match-beginning 2))
-	    (contents-begin (or (match-beginning 4)
-				(match-beginning 3)))
-	    (contents-end (or (match-end 4) (match-end 3)))
-	    (post-blank (progn (goto-char (match-end 0))
-			       (skip-chars-forward " \t")))
-	    (end (point)))
-	(org-element-create
-         'subscript
-	 (list :begin begin
-	       :end end
-	       :use-brackets-p bracketsp
-	       :contents-begin contents-begin
-	       :contents-end contents-end
-	       :post-blank post-blank))))))
+    (unless (bolp)
+      (backward-char)
+      (when (looking-at org-match-substring-regexp)
+        (let ((bracketsp (if (match-beginning 4) t nil))
+	      (begin (match-beginning 2))
+	      (contents-begin (or (match-beginning 4)
+				  (match-beginning 3)))
+	      (contents-end (or (match-end 4) (match-end 3)))
+	      (post-blank (progn (goto-char (match-end 0))
+			         (skip-chars-forward " \t")))
+	      (end (point)))
+	  (org-element-create
+           'subscript
+	   (list :begin begin
+	         :end end
+	         :use-brackets-p bracketsp
+	         :contents-begin contents-begin
+	         :contents-end contents-end
+	         :post-blank post-blank)))))))
 
 (defun org-element-subscript-interpreter (subscript contents)
   "Interpret SUBSCRIPT object as Org syntax.

testing/lisp/test-org-element.el

@@ -3219,7 +3219,13 @@ Outside list"
      (org-test-with-temp-text "_first line\nsecond line_"
        (org-element-map
 	   (org-element-parse-buffer) 'underline #'identity nil t)))
-    '("first line\nsecond line"))))
+    '("first line\nsecond line")))
+  ;; Nested underlines.
+  (should
+   (= 2
+      (org-test-with-temp-text "__test__"
+	(length
+	 (org-element-map (org-element-parse-buffer) 'underline 'identity))))))
 
 
 ;;;; Verbatim