typedef CHANGEME DATA;

typedef struct stack {
	DATA d;
	struct stack *next;
} ELEMENT;

typedef ELEMENT *TOP;

int isempty(TOP);  /* empty in stack.adt */
DATA top(TOP);		/* top in stack.adt*/
void pop(TOP*, DATA*);	/* after pop(&S, &i) S'=pop(S) and i'=top(S)*/
void push( TOP *t, DATA x);  /*After push(&S, x)  S'=push(S,x). */