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 _plot_surface(ax, x, y, z, labels, azim=None):
    """helper function for surface plots"""
    # ax.tick_params(axis='both', which='major', pad=-3)
    assert np.size(x) > 1 and np.size(y) > 1 and np.size(z) > 1
    if azim is not None:
        ax.azim = azim
    X, Y = np.meshgrid(x, y)
    Z = np.ma.masked_invalid(z)
    ax.plot_surface(X, Y, Z,
                    rstride=1, cstride=1,
                    cmap=cm.viridis, alpha=0.85,
                    vmin=np.nanmin(z), vmax=np.nanmax(z),
                    linewidth=0, antialiased=True)