symdiff: new function.

* eval.c (eval_init): Register symdiff intrinsic.

* lib.c (symdiff): New function.

* lib.h (us_car_p, us_cdr_p): New inline functions.
(symdiff): Declared.

* txr.1: Documented, also fixing issues not related to
symdiff doc.

1 files changed, 23 insertions, 9 deletions

diff --git a/txr.1 b/txr.1
index 396019b1..1799bfba 100644
--- a/txr.1
+++ b/txr.1
@@ -28702,31 +28702,32 @@ the leftmost one is reported. See the notes under
 .code pos-max
 regarding duplicate maxima.
 
-.coNP Functions @, uni @ isec and @ diff
+.coNP Functions @, uni @, isec @ diff and @ symdiff
 .synb
 .mets (uni < seq1 < seq2 >> [ testfun <> [ keyfun ]])
 .mets (isec < seq1 < seq2 >> [ testfun <> [ keyfun ]])
 .mets (diff < seq1 < seq2 >> [ testfun <> [ keyfun ]])
+.mets (symdiff < seq1 < seq2 >> [ testfun <> [ keyfun ]])
 .syne
 .desc
 The functions
-.codn diff ,
-.code isec
+.codn uni ,
+.codn isec ,
+.code diff
 and
-.code uni
-treats the sequences
+.code symdiff
+treat the sequences
 .meta seq1
 and
 .meta seq2
 as if they were sets.
 
-They respectively compute the set union, set intersection
-and set difference of
+They, respectively, compute the set union, set intersection,
+set difference and symmetric difference of
 .meta seq1
 and
 .metn seq2 ,
-returning a new sequence: the union sequence,
-intersection sequence and difference sequence.
+returning a new sequence.
 
 The arguments
 .meta seq1
@@ -28807,6 +28808,19 @@ If an element occurs at least once in
 .metn seq2 ,
 then it does not occur in the difference sequence.
 
+The symmetric difference sequence produced by
+.code symdiff
+contains all of the elements of
+.meta seq1
+which do not occur in
+.meta seq2
+and
+.IR "vice versa" :
+it also contains all of the elements of
+.meta seq2
+which do not occur in
+.metn seq1 .
+
 Element equivalence is determined by a combination of
 .meta testfun
 and