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_bad_isointervals():
    bad_intervals = [
        '2013-01T14:',
        '',
        'asdf',
        '01/01/2013',
    ]

    for bad_interval in bad_intervals:
        yield (
            assert_raises,
            Exception,
            inputs.iso8601interval,
            bad_interval,
        )