Add conditional operator xor to subr.el

Suggested by Oleh Krehel and implemented by Basil Contovounesios in
the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-07/msg00547.html

* lisp/array.el (xor): Move unused function from here...
* lisp/subr.el: ...to here, and improve.
* lisp/gnus/spam.el (spam-xor):
* lisp/play/5x5.el (5x5-xor):
* lisp/proced.el (proced-xor):
* lisp/progmodes/idlwave.el (idlwave-xor):
* lisp/vc/diff-mode.el (diff-xor): Define as obsolete aliases of,
and replace all uses with, xor.
* lisp/jsonrpc.el: Remove unused dependency on array.el.
* lisp/org/org.el (org-xor): Move from here...
* lisp/org/org-compat.el (org-xor): ...to here, as a compatibility
shim for xor.
* lisp/progmodes/idlw-shell.el (idlwave-shell-enable-all-bp):
* lisp/simple.el (exchange-point-and-mark):
* lisp/windmove.el (windmove-display-in-direction): Use xor.
* lisp/strokes.el (strokes-xor): Remove commented-out xor
implementation.

* doc/lispref/control.texi (Control Structures): Extend menu entry
for new combining condition.
(Combining Conditions):
* etc/NEWS (Lisp Changes): Document xor.

* test/lisp/subr-tests.el (subr-test-xor): New test.

1 files changed, 7 insertions, 0 deletions

diff --git a/test/lisp/subr-tests.el b/test/lisp/subr-tests.el
index 0023680738d..b3c04cdc9a8 100644
--- a/test/lisp/subr-tests.el
+++ b/test/lisp/subr-tests.el
@@ -125,6 +125,13 @@
   (should (equal (macroexpand-all '(when a b c d))
                  '(if a (progn b c d)))))
 
+(ert-deftest subr-test-xor ()
+  "Test `xor'."
+  (should-not (xor nil nil))
+  (should (eq (xor nil 'true) 'true))
+  (should (eq (xor 'true nil) 'true))
+  (should-not (xor t t)))
+
 (ert-deftest subr-test-version-parsing ()
   (should (equal (version-to-list ".5") '(0 5)))
   (should (equal (version-to-list "0.9 alpha1") '(0 9 -3 1)))