Prolog: splitting a list into two lists (unique items / duplicate items) -

i have been trying split given list 2 different lists: unique , duplicate. example, if have list [1, 1, 2, 3, 3, 4, 5] want unique list [2, 4, 5] , duplicate [1, 3]. don't want 1's in list in duplicate list. need 1 of it. code have right now:

comparel([_|[]], unique, dup).     comparel([x3,y3 | tail], [x3 | unique], dup) :-     x3 =\= y3,     comparel([y3 | tail], unique, dup).  comparel([x3,y3 | tail], unique, [x3 | dup]) :-      x3 = y3,     skipdups(x3, tail, unique, dup).  skipdups(_, [], unique, dup).    skipdups(x3,[y3 | tail], unique, dup) :-      x3 =\= y3,     comparel([y3 | tail], unique, dup). skipdups(x3,[y3 | tail], unique, dup) :-     x3 = y3,     skipdups(x3, tail, unique, dup). 

using example list given above if run comparel([1, 1, 2, 3, 3, 4, 5], unique, dup). get:

unique = [2, 4|_g1954], dup = [1, 3|_g1948]. 

i can't figure out why towards end of both lists getting '_g1954' , '_g1948'. appreciated. thanks.

here solution, key take/4 consumes matching leading items, enabling easy testing of list ( [_|_] matches list of @ least 1 element )

comparel([], [], []). comparel([x|xs], u, d) :-     (   take(x, xs, [_|_], ys)     ->  comparel(ys, u, b), d = [x|b]     ;   comparel(xs, a, d), u = [x|a]     ).  take(x, [x|xs], [x|r], ys) :-     !, take(x, xs, r, ys). take(_, ys, [], ys).