docs: def Definition(self):
      """Return a string with the definition of this field."""
      result = self._FormatDescriptionComment()
  
      result += "  enum %s {\n" % self.enum_name
      for k, v in sorted(iteritems(self.reverse_enum)):
        result += "    %s = %s;\n" % (v, k)
  
      result += "  }\n"
  
      result += self._FormatField()
      return result