def add(self, elem, replace=False):
        """Add a requirement.

        :param elem: a string or :class:`Requirement` instance.
        :param replace: if `True`, packages in the set with the same name will
            be removed first.
        """
        if isinstance(elem, text_type):
            elem = Requirement.coerce(elem)
        if replace and elem in self:
            self.remove(elem)
        super(Requirements, self).add(elem)
def test_abstract_class(self):
        class X(Structure):
            _abstract_ = "something"
        # try 'X()'
        cls, msg = self.get_except(eval, "X()", locals())
        self.assertEqual((cls, msg), (TypeError, "abstract class"))