docs: def to_xml(cls, enum_val):
          """
          Return the XML value of the enumeration value *enum_val*.
          """
          if enum_val not in cls._member_to_xml:
              raise ValueError(
                  "value '%s' not in enumeration %s" % (enum_val, cls.__name__)
              )
          return cls._member_to_xml[enum_val]