def delete(self, doc):
        if doc.id is None:
            raise DatabaseException('Document does not have id')
        if doc.sid is None:
            try:
                rs = self.db.docs.remove({'id': doc.id}, safe=True)
                assert rs['n'] == 1
            except Exception as e:
                raise DatabaseException(e)
            return None
        doc.data = None
        return self.update(doc)
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)