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 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)