add flatten.

README.md

@@ -136,6 +136,7 @@ The following predicates are built-in to rusty-wam.
 * `display/1`
 * `duplicate_term/2`
 * `false/0`
+* `flatten/2`
 * `float/1`
 * `functor/3`
 * `ground/1`

src/prolog/lib/lists.pl

@@ -1,5 +1,5 @@
 :- module(lists, [member/2, select/3, append/3, is_list/1, memberchk/2, reverse/2, maplist/2,
-		  maplist/3, maplist/4, maplist/5, maplist/6, maplist/7, maplist/8,
+		  flatten/2, maplist/3, maplist/4, maplist/5, maplist/6, maplist/7, maplist/8,
 		  maplist/9]).
 
 member(X, [X|_]).
@@ -11,7 +11,7 @@ select(X, [Y|Xs], [Y|Ys]) :- select(X, Xs, Ys).
 append([], R, R).
 append([X|L], R, [X|S]) :- append(L, R, S).
 
-is_list(X) :- var(X), !, false.        
+is_list(X) :- var(X), !, false.
 is_list([]).
 is_list([_|T]) :- is_list(T).
 
@@ -24,6 +24,14 @@ reverse([], Ys, Ys).
 reverse([H|T], Ps, Rs) :-
     reverse(T, [H|Ps], Rs).
 
+flatten([], List, List) :- !.
+flatten([Head|Tail], List0, List) :- !,
+    flatten(Head, List0, List1),
+    flatten(Tail, List1, List).
+flatten(Other, [Other|List], List).
+
+flatten(Tree, List) :- flatten(Tree, List, []).
+
 maplist(_, []).
 maplist(Cont1, [E1|E1s]) :-
     call(Cont1, E1),