def METHOD_NAME(self):
    """
    The index of this basis element.
    EXAMPLES::
        sage: # needs sage.combinat sage.modules
        sage: V = lie_conformal_algebras.NeveuSchwarz(QQ)
        sage: V.inject_variables()
        Defining L, G, C
        sage: G.T(3).index()
        ('G', 3)
        sage: v = V.an_element(); v
        L + G + C
        sage: v.index()
        Traceback (most recent call last):
        ...
        ValueError: index can only be computed for monomials, got L + G + C
    """
    if self.is_zero():
        return None
    if not self.is_monomial():
        raise ValueError("index can only be computed for "
                         "monomials, got {}".format(self))
    return next(iter(self.monomial_coefficients()))