is_odd(X) :- 1 is X mod 2.

part( _Predicate, [], [], [] ).             % if no elements we produce two empty lists
part( Predicate, [H|T], [H|T1], T2 ) :-
    apply(Predicate,[H]),                   % if the H satifies Predicate
    part(Predicate, T, T1, T2).             % H is added in front of first list
part( Predicate, [H|T], T1, [H|T2] ) :-
    not(apply(Predicate,[H])),              % else
    part(Predicate, T, T1, T2).             % is added to the second list

square(N, N2) :-
    nonvar(N),
    N2 is N*N.
square(N,N2) :-
    var(N),
    between(0,N2,N),
    N2 is N*N.