package body stacks is

	type stack_node is record 
			head:Item;
			rest:stack;
		end record;

	function newstack return Stack is
	begin
		return NULL;
	end newstack;

	function pop( the_stack:Stack) return Stack is
		the_head:Stack;
	begin
		if empty(the_stack) then
			raise stack_empty;
		else
			return the_stack.rest;
		end if;
	end pop;

	function push( the_stack:Stack; an_item:Item) return Stack is
	begin
		return new stack_node'(head=>an_item, rest=>the_stack);	
	end push;

	function top( the_stack: Stack) return Item is
	begin
		if empty(the_stack) then
			raise stack_empty;
		else
			return the_stack.head;
		end if;
	end top;

	function empty( the_stack:Stack) return Boolean is
	begin
		return the_stack=NULL;
	end empty;
	
end stacks;