Return a sequence containing the enumeration values that are valid
        assignment values. Return-only values are excluded.

def _collect_valid_settings(meta, clsdict):
        """
        Return a sequence containing the enumeration values that are valid
        assignment values. Return-only values are excluded.
        """
        enum_members = clsdict['__members__']
        valid_settings = []
        for member in enum_members:
            valid_settings.extend(member.valid_settings)
        clsdict['_valid_settings'] = valid_settings