add subtract/3 to lists.

docs/yap.tex

@@ -1,4 +1,4 @@
-<EFBFBD>a\input texinfo @c -*- mode: texinfo; coding: latin-1; -*-
+\input texinfo @c -*- mode: texinfo; coding: latin-1; -*-
 
 @c %**start of header
 @setfilename yap.info
@@ -9242,6 +9242,18 @@ Succeeds if @var{Set3} unifies with the intersection of @var{Set1} and
 need not be ordered.
 @end table
 
+@item subtract(+@var{Set}, +@var{Delete}, ?@var{Result})
+@findex subtract/3
+@syindex subtract/3
+@cnindex subtract/3
+Delete all elements from @var{Set} that   occur  in @var{Delete} (a set)
+and unify the  result  with  @var{Result}.   Deletion  is  based  on
+unification using @code{memberchk/2}. The complexity is
+@code{|Delete|*|Set|}.
+
+See @code{ord_subtract/3}.
+@end table
+
 @node LineUtilities, MapList, Lists, Library
 @section Line Manipulation Utilities
 @cindex Line Utilities Library

library/lists.yap

@@ -8,9 +8,14 @@
 	   append/3,
 	   append/2,
 	   delete/3,
+	   intersection/3,
+	   flatten/2,
 	   last/2,
+	   list_concat/2,
+	   max_list/2,	 
 	   member/2,
 	   memberchk/2,
+	   min_list/2,
 	   nextto/3,
 	   nth/3,
 	   nth/4,
@@ -18,6 +23,7 @@
 	   nth0/4,
 	   nth1/3,
 	   nth1/4,
+	   numlist/3,
 	   permutation/2,
 	   prefix/2,
 	   remove_duplicates/2,
@@ -27,16 +33,11 @@
 	   selectchk/3,
 	   sublist/2,
 	   substitute/4,
+	   subtract/3,
+	   suffix/2,
 	   sum_list/2,
 	   sum_list/3,
-	   suffix/2,
-	   sumlist/2,
-	   list_concat/2,
-	   flatten/2,
-	   max_list/2,	 
-	   min_list/2,
-	   numlist/3,
-	   intersection/3
+	   sumlist/2
 	  ]).
 
 :- use_module(library(error),
@@ -400,3 +401,17 @@ intersection([X|T], L, Intersect) :-
 intersection([_|T], L, R) :-
 	intersection(T, L, R).
 
+%%	subtract(+Set, +Delete, -Result) is det.
+%
+%	Delete all elements from `Set' that   occur  in `Delete' (a set)
+%	and unify the  result  with  `Result'.   Deletion  is  based  on
+%	unification using memberchk/2. The complexity is |Delete|*|Set|.
+%
+%	@see ord_subtract/3.
+
+subtract([], _, []) :- !.
+subtract([E|T], D, R) :-
+	memberchk(E, D), !,
+	subtract(T, D, R).
+subtract([H|T], D, [H|R]) :-
+	subtract(T, D, R).