bug#13989: Make Info support footnotes

[Juri Linkov <juri@jurta.org>]

> It would be nice if Info supports footnotes, just like
> `org-footnote-action' in Org:
> (1) When at a footnote reference, jump to the definition.
> (2) When at a definition, jump to the references if they exist, offer
> to create them otherwise.
>
> Or another choice: make footnote.el support Info.

Thanks for the suggestion.  Footnotes like in Org and Wikipedia
would be nice to have.  And it's pretty straightforward to implement:

=== modified file 'lisp/info.el'
--- lisp/info.el	2013-02-21 06:55:19 +0000
+++ lisp/info.el	2013-03-18 22:34:07 +0000
@@ -3840,7 +3861,21 @@ (defun Info-try-follow-nearest-node (&op
      ((setq node (Info-get-token (point) "File: " "File: \\([^,\n\t]*\\)"))
       (Info-goto-node "Top" fork))
      ((setq node (Info-get-token (point) "Prev: " "Prev: \\([^,\n\t]*\\)"))
-      (Info-goto-node node fork)))
+      (Info-goto-node node fork))
+     ;; footnote
+     ((setq node (Info-get-token (point) "(" "\\(([0-9]+)\\)"))
+      (let ((old-point (point)) new-point)
+	(save-excursion
+	  (goto-char (point-min))
+	  (when (re-search-forward "^[ \t]*-+ Footnotes -+$" nil t)
+	    (setq new-point (if (< old-point (point))
+				;; Go to footnote reference
+				(search-forward node nil t)
+			      ;; Go to footnote definition
+			      (search-backward node nil t)))))
+	(when new-point
+	  (goto-char new-point)
+	  (setq node t)))))
     node))
 
 (defun Info-mouse-follow-link (click)
@@ -4896,6 +4931,20 @@ (defun Info-fontify-node ()
                                  mouse-face highlight
                                  help-echo "mouse-2: go to this URL"))))
 
+      ;; Fontify footnotes
+      (goto-char (point-min))
+      (when (and not-fontified-p (re-search-forward "^[ \t]*-+ Footnotes -+$" nil t))
+	(let ((limit (point)))
+	  (goto-char (point-min))
+	  (while (re-search-forward "\\(([0-9]+)\\)" nil t)
+	    (add-text-properties (match-beginning 0) (match-end 0)
+                                 `(font-lock-face info-xref
+                                   mouse-face highlight
+                                   help-echo
+				   ,(if (< (point) limit)
+					"mouse-2: go to footnote definition"
+				      "mouse-2: go to footnote reference"))))))
+
       ;; Hide empty lines at the end of the node.
       (goto-char (point-max))
       (skip-chars-backward "\n")