def coerce(cls, string):
        """Create a :class:`Requirement` object from a given package spec."""
        match = re.match(cls.spec_regex, string)
        if not match:
            raise InvalidRequirementSpecError("could not parse requirement")
        package = match.group(1)
        if all(match.group(2, 3)):
            version = "".join(match.group(2, 3))
        else:
            version = None
        return cls(package, version)
def i1beta_ld(i1, beta, ld, ax=0):
    """creates a surface plot of ld vs i1, beta"""
    if ax == 0:
        _create_3d_axis()
        ax = plt.gca()
    _plot_surface(ax, i1, beta, np.asarray(ld)*1e3,
                  (u'I1/A', u'Beta/°', u'Ld/mH'),
                  azim=60)