code: func (h *TermsHashPerFieldImpl) start(field IndexableField, first bool) bool {
  	if h.termAtt = h.fieldState.termAttribute; h.termAtt != nil {
  		// EmptyTokenStream can have nil term att
  		h.termBytesRef = h.termAtt.BytesRef()
  	}
  	if h.nextPerField != nil {
  		h.doNextCall = h.nextPerField.start(field, first)
  	}
  	return true
  }
docs: /*
  Start adding a new field instance; first is true if this is the first
  time this field name was seen in the document.
  */