def __init__(self, package, version):
        """Construct a new requirement.

        :param package: the package name.
        :param version: a semver compatible version specification.
        """
        self.package = package
        self.version = version
        if self.version and not re.match(r"[<=>~]", version[:2]):
            self.version = "=={}".format(self.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)