/* File: stack0.pl     System: SWI-Prolog  5.2.8
   Author: Mihaela Malita
   Date: 10/23/2003
   Title:  A STACK in Prolog
    ?- push(a).
    Yes
    ?- push(b).
    Yes
    ?- push(c).
    Yes
    ?- print_stack.
    Stack is: [c, b, a]
    ?- pop(I).
    I = c 
    ?- look(X).
    X = b
    ?- pop(I).
    I = b 
    ?- pop(I).
    I = a 
    ?- empty_stack.
    Yes
    ?- print_stack.
    Stack is: []
*************************************************/
% declare stack as "dynamic" so you can change it

    :-dynamic(stack/1).         

% intially the stack is empty.

    stack([]).

% push something H in your stack. Remove old stack and put new stack instead

    push(H):- stack(L),New=[H|L],retract(stack(_)),assert(stack(New)).

% pop something out of the stack

    pop(H):- stack([H|L]),retract(stack(_)),assert(stack(L)).

% check if stack is empty

    empty_stack:- stack([]).

% look at the top of the stack. Do not do anything.

  look(Top):- stack([Top|_]).
  
% print the stack

    print_stack:- stack(L),write('Stack is: '),write(L).