# Lesson XVIII The hand may hold nothing. See the empty hand. Do not ask it for a line. ## Try ```prolog ?- E = kernel, run(E, [push(nil), dup, eq], [], Stack). Stack = [bool(true)]. ?- E = kernel, infer(E, [push(nil), dup, eq], Scheme). Scheme = scheme([], effect(S, [bool|S])). ?- E = kernel, infer(E, [push(nil), dup, add], Scheme). ERROR: forth_error(constraint, unsatisfied, requires(numeric, nil)). ``` ## Lesson ```prolog unpack(nil, nil, nil) :- !. pack(nil, nil, nil) :- !. lit(_Env, nil, nil). supports(nil, equatable). ``` [[17_lesson|Prev]] | Next