code: func (h *postHeap) next() postEntry {
  	if len(h.ch) == 0 {
  		return makePostEntry(1<<24-1, 0)
  	}
  	ch := h.ch[0]
  	e := ch.e
  	m := ch.m
  	if len(m) == 0 {
  		h.pop()
  	} else {
  		ch.e = m[0]
  		ch.m = m[1:]
  		h.siftDown(0)
  	}
  	return e
  }
docs: // next returns the next entry from the postHeap.
  // It returns a postEntry with trigram == 1<<24 - 1 if h is empty.