code: func (st *state) refQualifier() string {
  	if len(st.str) > 0 {
  		switch st.str[0] {
  		case 'R':
  			st.advance(1)
  			return "&"
  		case 'O':
  			st.advance(1)
  			return "&&"
  		}
  	}
  	return ""
  }
docs: // <ref-qualifier> ::= R
  //                 ::= O