code: func (h *Hasher) ResetWithLength(span []byte) {
  	h.Reset()
  	h.getTree().span = span
  }
docs: // methods needed to implement the SwarmHash interface
  // ResetWithLength needs to be called before writing to the hasher
  // the argument is supposed to be the byte slice binary representation of
  // the length of the data subsumed under the hash, i.e., span