/*
 *  call-seq:
 *     pq.pop -> elem
 *  
 *  Returns the top element in the queue removing it from the queue.
 */
static VALUE
frt_pq_pop(VALUE self)
{
    PriQ *pq;
    GET_PQ(pq, self);
    if (pq->size > 0) {
        VALUE result = pq->heap[1];       /* save first value */
        pq->heap[1] = pq->heap[pq->size]; /* move last to first */
        pq->heap[pq->size] = Qnil;
        pq->size--;
        pq_down(pq);                      /* adjust heap */
        return result;
    }
    else {
        return Qnil;
    }
}