void Pop (Stack *S) {
    StackNode *p, *q;

    if (S->size == 1) {
        q = S->first;
        S->first = NULL;
    }
    else {
        q=S->first;
        while(q->next != NULL)
            q = q->next;
    }
    printf("popping = %d\n", q->item);


    destroyItem(&q->item);
    q = NULL;
    free(q);
    S->size--;
}