ox-texinfo: Support lettered lists

* lisp/ox-texinfo.el (org-texinfo-plain-list): Add :enum attribute.
* doc/org-manual.org (Plain lists in Texinfo export): Document :enum
  attribute.

doc/org-manual.org

@@ -14773,21 +14773,22 @@ This paragraph is preceded by...
 
 #+cindex: @samp{ATTR_TEXINFO}, keyword
 #+cindex: two-column tables, in Texinfo export
-
-#+cindex: table types, in Texinfo export
+#+cindex: table-type, Texinfo attribute
 The Texinfo export back-end by default converts description lists in
 the Org file using the default command =@table=, which results in
-a table with two columns.  To change this behavior, specify
-=:table-type= with =ftable= or =vtable= attributes.  For more
-information, see [[info:texinfo::Two-column Tables]].
+a table with two columns.  To change this behavior, set =:table-type=
+attribute to either =ftable= or =vtable= value.  For more information,
+see [[info:texinfo::Two-column Tables]].
 
 #+vindex: org-texinfo-table-default-markup
+#+cindex: indic, Texinfo attribute
 The Texinfo export back-end by default also applies a text highlight
 based on the defaults stored in ~org-texinfo-table-default-markup~.
 To override the default highlight command, specify another one with
 the =:indic= attribute.
 
 #+cindex: multiple items in Texinfo lists
+#+cindex: sep, Texinfo attribute
 Org syntax is limited to one entry per list item.  Nevertheless, the
 Texinfo export back-end can split that entry according to any text
 provided through the =:sep= attribute.  Each part then becomes a new
@@ -14811,6 +14812,20 @@ This is the common text for variables foo and bar.
 @end table
 #+end_example
 
+#+cindex: lettered lists, in Texinfo export
+#+cindex: enum, Texinfo attribute
+Ordered lists are numbered when exported to Texinfo format.  Such
+numbering obeys any counter (see [[*Plain Lists]]) in the first item of
+the list.  The =:enum= attribute also let you start the list at
+a specific number, or switch to a lettered list, as illustrated here
+
+#+begin_example
+#+ATTR_TEXINFO: :enum A
+1. Alpha
+2. Bravo
+3. Charlie
+#+end_example
+
 *** Tables in Texinfo export
 :PROPERTIES:
 :DESCRIPTION: Table attributes.

etc/ORG-NEWS

@@ -83,6 +83,11 @@ since now it's implicitly always true.
 
 This aligns Beamer notes with slide overlays.
 
+*** Add support for lettered lists in Texinfo
+
+Using =:enum A= or =:enum a= Texinfo attribute switches an otherwise
+numbered list to a lettered list.
+
 *** Add a dispatcher command to insert dynamic blocks
 
 You can add dynamic block into ~org-dynamic-block-alist~ with function

lisp/ox-texinfo.el

@@ -1253,12 +1253,14 @@ contextual information."
 		  (if (string-prefix-p "@" i) i (concat "@" i))))
 	 (table-type (plist-get attr :table-type))
 	 (type (org-element-property :type plain-list))
-	 (initial-counter
-	  (and (eq type 'ordered)
+	 (enum
+	  (cond ((not (eq type 'ordered)) nil)
+		((plist-member attr :enum) (plist-get attr :enum))
+		(t
 		 ;; Texinfo only supports initial counters, i.e., it
 		 ;; cannot change the numbering mid-list.
 		 (let ((first-item (car (org-element-contents plain-list))))
-		 (org-element-property :counter first-item))))
+		   (org-element-property :counter first-item)))))
 	 (list-type (cond
 		     ((eq type 'ordered) "enumerate")
 		     ((eq type 'unordered) "itemize")
@@ -1266,7 +1268,7 @@ contextual information."
 		     (t "table"))))
     (format "@%s\n%s@end %s"
 	    (cond ((eq type 'descriptive) (concat list-type " " indic))
-		  (initial-counter (format "%s %d" list-type initial-counter))
+		  (enum (format "%s %s" list-type enum))
 		  (t list-type))
 	    contents
 	    list-type)))