fix unexpected error in length/2
@@ -54,7 +54,7 @@ length(Xs0, N) :-
'$skip_max_list'(M, N, Xs0,Xs),
!,
( Xs == [] -> N = M
- ; nonvar(Xs) -> var(N), throw(error(resource_error(finite_memory),length/2))
+ ; nonvar(Xs) -> var(N), Xs = [_|_], throw(error(resource_error(finite_memory),length/2))
; nonvar(N) -> R is N-M, length_rundown(Xs, R)
; N == Xs -> throw(error(resource_error(finite_memory),length/2))
; length_addendum(Xs, N, M)
