ox: Implement "notoc" UNNUMBERED value

* lisp/ox.el (org-export-collect-headlines): Exclude headlines with
  UNNUMBERED property set to "notoc".

* doc/org.texi (Export settings):
(Table of contents): Document new value.

* testing/lisp/test-ox.el (test-org-export/collect-headlines): Add
  test.

doc/org.texi

@@ -10813,7 +10813,9 @@ Toggle inclusion of inlinetasks (@code{org-export-with-inlinetasks}).
 Toggle section-numbers (@code{org-export-with-section-numbers}).  When set to
 number @samp{n}, Org numbers only those headlines at level @samp{n} or above.
 Set @code{UNNUMBERED} property to non-@code{nil} to disable numbering of
-heading and subheadings entirely.
+heading and subheadings entirely.  Moreover, when the value is @samp{notoc}
+the headline, and all its children, do not appear in the table of contents
+either (@pxref{Table of contents}).
 
 @item p:
 @vindex org-export-with-planning
@@ -10911,6 +10913,22 @@ keyword:
 #+OPTIONS: toc:nil        @r{no default TOC at all}
 @end example
 
+@cindex excluding entries from table of contents
+@cindex table of contents, exclude entries
+Org includes both numbered and unnumbered headlines in the table of
+contents@footnote{At the moment, some export back-ends do not obey this
+specification.  For example, @LaTeX{} export excludes every unnumbered
+headline from the table of contents.}.  If you need to exclude an unnumbered
+headline, along with all its children, set the @samp{UNNUMBERED} property to
+@samp{notoc} value.
+
+@example
+* Subtree not numbered, not in table of contents either
+  :PROPERTIES:
+  :UNNUMBERED: notoc
+  :END:
+@end example
+
 @cindex #+TOC
 Org normally inserts the table of contents directly before the first headline
 of the file.  To move the table of contents to a different location, first
@@ -10935,8 +10953,8 @@ compatibility issues, @code{titletoc} has to be loaded @emph{before}
 variable.
 
 @example
-* Section #+TOC: headlines 1 local @r{insert local TOC, with direct children
-only}
+* Section
+#+TOC: headlines 1 local @r{insert local TOC, with direct children only}
 @end example
 
 Use the @code{TOC} keyword to generate list of tables (resp.@: all listings)

etc/ORG-NEWS

@@ -26,6 +26,9 @@ document, use =shrink= value instead, or in addition to align:
 #+END_EXAMPLE
 
 ** New features
+*** Exclude unnumbered headlines from table of contents
+Set their =UNNUMBERED= property to the special =notoc= value.  See
+manual for details.
 *** ~org-archive~ functions update status cookies
 
 Archiving headers through ~org-archive-subtree~ and

lisp/ox.el

@@ -5226,12 +5226,16 @@ Footnote sections are ignored."
 	 (n (if (not (wholenump n)) limit
 	      (min (if (eq (org-element-type scope) 'org-data) n
 		     (+ (org-export-get-relative-level scope info) n))
-		   limit))))
+		   limit)))
+	 (skipped nil))
     (org-element-map (org-element-contents scope) 'headline
-      (lambda (headline)
-	(unless (org-element-property :footnote-section-p headline)
-	  (let ((level (org-export-get-relative-level headline info)))
-	    (and (<= level n) headline))))
+      (lambda (h)
+	(if (or (org-element-property :footnote-section-p h)
+		(equal "notoc" (org-element-property :UNNUMBERED h))
+		(memq (org-element-property :parent h) skipped)
+		(< n (org-export-get-relative-level h info)))
+	    (progn (push h skipped) nil)
+	  h))
       info)))
 
 (defun org-export-collect-elements (type info &optional predicate)

testing/lisp/test-ox.el

@@ -4314,6 +4314,21 @@ Another text. (ref:text)
 	    (org-test-with-parsed-data "* H1\n** Footnotes"
 	      (mapcar (lambda (h) (org-element-property :raw-value h))
 		      (org-export-collect-headlines info))))))
+  ;; Do not collect headlines with UNNUMBERED property set to "notoc".
+  ;; Headlines with another value for the property are still
+  ;; collected.
+  (should
+   (equal '("H1")
+	  (org-test-with-parsed-data
+	      "* H1\n* H2\n:PROPERTIES:\n:UNNUMBERED: notoc\n:END:"
+	    (mapcar (lambda (h) (org-element-property :raw-value h))
+		    (org-export-collect-headlines info)))))
+  (should
+   (equal '("H1" "H2")
+	  (org-test-with-parsed-data
+	      "* H1\n* H2\n:PROPERTIES:\n:UNNUMBERED: t\n:END:"
+	    (mapcar (lambda (h) (org-element-property :raw-value h))
+		    (org-export-collect-headlines info)))))
   ;; Collect headlines locally.
   (should
    (equal '("H2" "H3")