code: def _check_utf8_encoding(self, bstring):
          
          if not gf.is_bytes(bstring):
              self._failed(u"The given string is not a sequence of bytes")
              return
          if not gf.is_utf8_encoded(bstring):
              self._failed(u"The given string is not encoded in UTF-8.")
docs: Check whether the given sequence of bytes
          is properly encoded in UTF-8.
  
          :param bytes bstring: the byte string to be checked