code: func (h *postHeap) add(ch *postChunk) {
  	if len(ch.m) > 0 {
  		ch.e = ch.m[0]
  		ch.m = ch.m[1:]
  		h.push(ch)
  	}
  }
docs: // add adds the chunk to the postHeap.
  // All adds must be called before the first call to next.