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 mcv_muer(mcv, ax=0):
    """plot rel. permeability vs. B of mcv dict"""
    MUE0 = 4e-7*np.pi
    bi, ur = zip(*[(bx, bx/hx/MUE0)
                   for bx, hx in zip(mcv['curve'][0]['bi'],
                                     mcv['curve'][0]['hi']) if not hx == 0])
    if ax == 0:
        ax = plt.gca()
    ax.plot(bi, ur)
    ax.set_xlabel('B / T')
    ax.set_title('rel. Permeability')
    ax.grid()