code: def longest_one_seg_prefix(self, word):
          
          match = self.seg_regex.match(word)
          if match:
              return match.group(0)
          else:
              return ''
docs: Return longest IPA Unicode prefix of `word`
  
          Args:
              word (unicode): word as IPA string
  
          Returns:
              unicode: longest single-segment prefix of `word`