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 __get_linearForce_title_keys(lf):
    if 'force_r' in lf:
        return ['Force r', 'Force z'], ['force_r', 'force_z']
    return ['Force x', 'Force y'], ['force_x', 'force_y']