def identifiers(self, identifiers):
        """
        :type identifiers:  subject_abcs.IdentifierCollection
        """
        if (isinstance(identifiers, subject_abcs.IdentifierCollection) or
                identifiers is None):
            self._identifiers = identifiers
        else:
            raise ValueError('must use IdentifierCollection')
:type identifiers:  subject_abcs.IdentifierCollection