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 idq_psiq(id, iq, psiq, ax=0):
    """creates a surface plot of psiq vs id, iq"""
    if ax == 0:
        _create_3d_axis()
        ax = plt.gca()
    _plot_surface(ax, id, iq, psiq,
                  (u'Id/A', u'Iq/A', u'Psi q/Vs'),
                  azim=210)