def METHOD_NAME(self):
    r"""
    Return the index set for the Cartan type ``self``.
    The index set for all standard finite Cartan types is of the form
    `\{1, \ldots, n\}`. (See :mod:`~sage.combinat.root_system.type_I`
    for a slight abuse of this).
    EXAMPLES::
        sage: CartanType(['A', NN]).index_set()
        Non negative integer semiring
        sage: CartanType(['A', ZZ]).index_set()
        Integer Ring
    """
    return self.n