code: def _compress_hextets(cls, hextets):
          
          best_doublecolon_start = -1
          best_doublecolon_len = 0
          doublecolon_start = -1
          doublecolon_len = 0
          for index, hextet in enumerate(hextets):
              if hextet == '0':
                  doublecolon_len += 1
                  if doublecolon_start == -1:
                      # Start of a sequence of zeros.
                      doublecolon_start = index
                  if doublecolon_len > best_doublecolon_len:
                      # This is the longest sequence of zeros so far.
                      best_doublecolon_len = doublecolon_len
                      best_doublecolon_start = doublecolon_start
              else:
                  doublecolon_len = 0
                  doublecolon_start = -1
  
          if best_doublecolon_len > 1:
              best_doublecolon_end = (best_doublecolon_start +
                                      best_doublecolon_len)
              # For zeros at the end of the address.
              if best_doublecolon_end == len(hextets):
                  hextets += ['']
              hextets[best_doublecolon_start:best_doublecolon_end] = ['']
              # For zeros at the beginning of the address.
              if best_doublecolon_start == 0:
                  hextets = [''] + hextets
  
          return hextets
docs: Compresses a list of hextets.
  
          Compresses a list of strings, replacing the longest continuous
          sequence of "0" in the list with "" and adding empty strings at
          the beginning or at the end of the string such that subsequently
          calling ":".join(hextets) will produce the compressed version of
          the IPv6 address.
  
          Args:
              hextets: A list of strings, the hextets to compress.
  
          Returns:
              A list of strings.