% implementing stacks in Prolog is easy by using the data base
:-dynamic(stack_top/2).

new_stack(S):-retract(stack_top(S,_)), fail.
empty_stack(S):-not(stack_top(S,_)).
push(S,I):-asserta(stack_top(S, I)).
top(S,I):-stack_top(S,I), !.
pop(S,I):-retract(stack_top(S,I)).