include flatten/2 in library(lists).

git-svn-id: https://yap.svn.sf.net/svnroot/yap/trunk@598 b08c6af1-5177-4d33-ba66-4b1c6b8b522a

docs/yap.tex

@@ -6941,6 +6941,21 @@ True when @var{List} is a list, in which @var{Element} may or may not
 occur, and @var{Residue} is a copy of @var{List} with all elements
 identical to @var{Element} deleted.
 
+@item flatten(+@var{List}, ?@var{FlattenedList})
+@findex flatten/2
+@syindex flatten/2
+@cnindex flatten/2
+Flatten a list of lists @var{List} into a single list
+@var{FlattenedList}.
+
+@example
+?- flatten([[1],[2,3],[4,[5,6],7,8]],L).
+
+L = [1,2,3,4,5,6,7,8] ? ;
+
+no
+@end example
+
 @item is_list(+@var{List})
 @findex is_list/1
 @syindex is_list/1

library/lists.yap

@@ -25,7 +25,8 @@
 	sum_list/2,
 	suffix/2,
 	sumlist/2,
-        list_concat/2
+        list_concat/2,
+        flatten/2
 	]).
 
 
@@ -321,4 +322,12 @@ list_concat([H|T], [H|Lf], Li) :-
 
 
 
-
+%
+% flatten a list
+%
+flatten(X,Y) :- flatten_list(X,Y,[]).
+ 
+flatten_list([]) --> !.
+flatten_list([H|T]) --> !, flatten_list(H),flatten_list(T).
+flatten_list(H) --> [H].
+