org-export: Implement "ref" type links

* contrib/lisp/org-element.el (org-element-link-parser): Recognize
  "ref" type links as a new type of Org link.
* contrib/lisp/org-export.el (org-export-resolve-ref-link): New
  function.
* EXPERIMENTAL/org-e-ascii.el (org-e-ascii--describe-links): Do not
  describe "ref" type links at the end of each section.
(org-e-ascii-link): Handle "ref" type links.
* EXPERIMENTAL/org-e-latex.el (org-e-latex-link): Handle "ref" type
  links.

"ref" type links are Org answer to LaTeXisms like "\ref{some-label}".
Hence, [[ref:some-label]] will be replaced with the sequence number of
the element with name "#+name: some-label" during export.

EXPERIMENTAL/org-e-ascii.el

@@ -66,6 +66,7 @@
 (declare-function org-export-resolve-coderef "org-export" (ref info))
 (declare-function org-export-resolve-fuzzy-link "org-export" (link info))
 (declare-function org-export-resolve-id-link "org-export" (link info))
+(declare-function org-export-resolve-ref-link "org-export" (link info))
 (declare-function org-export-secondary-string
 		  "org-export" (secondary backend info))
 (declare-function org-export-table-format-info "org-export" (table))
@@ -833,8 +834,8 @@ channel."
 			 (org-element-get-property :raw-link link)
 		       (org-export-secondary-string desc 'e-ascii info)))))
        (cond
-	;; Coderefs and radio links are ignored.
-	((member type '("coderef" "radio")) nil)
+	;; Coderefs, radio links and ref links are ignored.
+	((member type '("coderef" "radio" "ref")) nil)
 	;; Id, custom-id and fuzzy links (with the exception of
 	;; targets): Headlines refer to their numbering.
 	((member type '("custom-id" "fuzzy" "id"))
@@ -1384,6 +1385,17 @@ INFO is a plist holding contextual information."
 	(org-element-get-property :path link)
 	(cdr (assq 'radio-target org-element-object-restrictions)))
        'e-ascii info))
+     ;; Ref link: If there's no description (DESC, return link's
+     ;; destination sequence number among elements of same
+     ;; type. Otherwise, use DESC.
+     ((string= type "ref")
+      (if (org-string-nw-p desc) desc
+	(format "%d"
+		(org-export-get-ordinal
+		 (org-export-resolve-ref-link link info)
+		 info nil nil
+		 (lambda (el) (or (org-element-get-property :caption el)
+			     (org-element-get-property :name el)))))))
      ;; Do not apply a special syntax on fuzzy links pointing to
      ;; targets.
      ((and (string= type "fuzzy")

EXPERIMENTAL/org-e-latex.el

@@ -1416,6 +1416,12 @@ INFO is a plist holding contextual information.  See
 	       (org-element-parse-secondary-string
 		path (cdr (assq 'radio-target org-element-object-restrictions)))
 	       'e-latex info)))
+     ;; Ref link: If no description is provided, reference label PATH
+     ;; and display table number.  Otherwise move to label but display
+     ;; description instead.
+     ((string= type "ref")
+      (if (not desc) (format "\\ref{%s}" path)
+	(format "\\hyperref[%s]{%s}" path desc)))
      ;; Links pointing to an headline: Find destination and build
      ;; appropriate referencing command.
      ((member type '("custom-id" "fuzzy" "id"))

contrib/lisp/org-element.el

@@ -1946,6 +1946,9 @@ Assume point is at the beginning of the link."
 	 ;; Explicit type (http, irc, bbdb...).  See `org-link-types'.
 	 ((string-match org-link-re-with-space3 link)
 	  (setq type (match-string 1 link) path (match-string 2 link)))
+	 ;; Ref type: PATH is the name of the target element.
+	 ((string-match "^ref:\\(.*\\)" link)
+	  (setq type "ref" path (org-trim (match-string 1 link))))
 	 ;; Id type: PATH is the id.
 	 ((string-match "^id:\\([-a-f0-9]+\\)" link)
 	  (setq type "id" path (match-string 1 link)))

contrib/lisp/org-export.el

@@ -2641,6 +2641,20 @@ is either \"id\" or \"custom-id\"."
          headline))
      info 'first-match)))
 
+(defun org-export-resolve-ref-link (link info)
+  "Return element referenced as LINK destination.
+
+INFO is a plist used as a communication channel.
+
+Assume LINK type is \"ref\" and.  Return value is the first
+element whose `:name' property matches LINK's `:path', or nil."
+  (let ((name (org-element-get-property :path link)))
+    (org-element-map
+     (plist-get info :parse-tree) org-element-all-elements
+     (lambda (el local)
+       (when (string= (org-element-get-property :name el) name) el))
+     info 'first-match)))
+
 (defun org-export-resolve-coderef (ref info)
   "Resolve a code reference REF.