code: func (h *postHeap) step(ch *postChunk) bool {
  	old := ch.e
  	m := ch.m
  	if len(m) == 0 {
  		return false
  	}
  	ch.e = postEntry(m[0])
  	m = m[1:]
  	ch.m = m
  	if old >= ch.e {
  		panic("bad sort")
  	}
  	return true
  }
docs: // step reads the next entry from ch and saves it in ch.e.
  // It returns false if ch is over.