def remove(self, elem):
        """Remove a requirement.

        :param elem: a string or :class:`Requirement` instance.
        """
        if isinstance(elem, text_type):
            for requirement in self:
                if requirement.package == elem:
                    return super(Requirements, self).remove(requirement)
        return super(Requirements, self).remove(elem)
def active_at_datetime(self, deadline):
        """Check whether the verification was active at a particular datetime.

        Arguments:
            deadline (datetime): The date at which the verification was active
                (created before and expiration datetime is after today).

        Returns:
            bool

        """
        return (
            self.created_at < deadline and
            self.expiration_datetime > now()
        )