ox: Plug citations into export process

* lisp/ox.el (org-export-as): Add citation pre-processing.

lisp/ox.el

@@ -74,6 +74,8 @@
 
 (require 'cl-lib)
 (require 'ob-exp)
+(require 'oc)
+(require 'oc-basic)    ;default value for `org-cite-export-processors'
 (require 'ol)
 (require 'org-element)
 (require 'org-macro)
@@ -140,7 +142,9 @@
     (:with-tasks nil "tasks" org-export-with-tasks)
     (:with-timestamps nil "<" org-export-with-timestamps)
     (:with-title nil "title" org-export-with-title)
-    (:with-todo-keywords nil "todo" org-export-with-todo-keywords))
+    (:with-todo-keywords nil "todo" org-export-with-todo-keywords)
+    ;; Citations processing.
+    (:cite-export "CITE_EXPORT" nil org-cite-export-processors))
   "Alist between export properties and ways to set them.
 
 The key of the alist is the property name, and the value is a list
@@ -2988,6 +2992,10 @@ Return code as a string."
 	 (setq info
 	       (org-combine-plists
 		info (org-export-get-environment backend subtreep ext-plist)))
+         ;; Pre-process citations environment, i.e. install
+	 ;; bibliography list, and citation processor in INFO.
+	 (org-cite-store-bibliography info)
+         (org-cite-store-export-processor info)
 	 ;; De-activate uninterpreted data from parsed keywords.
 	 (dolist (entry (append (org-export-get-all-options backend)
 				org-export-options-alist))
@@ -3021,6 +3029,11 @@ Return code as a string."
 	 ;; Now tree is complete, compute its properties and add them
 	 ;; to communication channel.
 	 (setq info (org-export--collect-tree-properties tree info))
+         ;; Process citations and bibliography.  Replace each citation
+	 ;; and "print_bibliography" keyword in the parse tree with
+	 ;; the output of the selected citation export processor.
+         (org-cite-process-citations info)
+         (org-cite-process-bibliography info)
 	 ;; Eventually transcode TREE.  Wrap the resulting string into
 	 ;; a template.
 	 (let* ((body (org-element-normalize-string
@@ -3033,16 +3046,19 @@ Return code as a string."
 			      (funcall inner-template body info))
 			    info))
 		(template (cdr (assq 'template
-				     (plist-get info :translate-alist)))))
+				     (plist-get info :translate-alist))))
+                (output
+                 (if (or (not (functionp template)) body-only) full-body
+	           (funcall template full-body info))))
+           ;; Call citation export finalizer.
+           (setq output (org-cite-finalize-export output info))
 	   ;; Remove all text properties since they cannot be
 	   ;; retrieved from an external process.  Finally call
 	   ;; final-output filter and return result.
 	   (org-no-properties
 	    (org-export-filter-apply-functions
 	     (plist-get info :filter-final-output)
-	     (if (or (not (functionp template)) body-only) full-body
-	       (funcall template full-body info))
-	     info))))))))
+	     output info))))))))
 
 ;;;###autoload
 (defun org-export-string-as (string backend &optional body-only ext-plist)