struct EE_STACK_NODE {
    double datum;
    struct EE_STACK_NODE* tail;
};
typedef struct EE_STACK_NODE NODE;
typedef struct EE_STACK_NODE* STACK;

STACK
push(double datum, STACK tail){
    /* Given a double value (datum), and a STACK (tail),
     * push(datum,tail) is the STACK resulting from
     * prepending datum to the STACK tail.
     */
     NODE* new_node = (NODE*)malloc(sizeof(NODE));
        (*new_node).datum = datum;
        (*new_node).tail  = tail;
    return new_node;
}

STACK
pop(STACK target_stack){
    /* Given a STACK (target_stack), pop(target_stack)
     * is the STACK resulting from the removal of the first
     * item in target_stack;
     */
     if (target_stack == NULL){ return target_stack; }
     STACK tail = (*target_stack).tail;
     free(target_stack);
     return tail;
}

double
peek(STACK target_stack){
    /* Given a STACK (target_stack), peek(STACK)
     * is the first element's datum.
     */
     if (target_stack == NULL){ return 0; }
     return (*target_stack).datum;
}