def METHOD_NAME(self):
    """
    Return an iterator for the keys (indices) of the family.
    EXAMPLES::
        sage: M = FiniteRankFreeModule(ZZ, 3, name='M')
        sage: T11 = M.tensor_module(1,1)
        sage: e11 = T11.basis('e')
        sage: list(e11.keys())
        [(0, 0), (0, 1), (0, 2),
         (1, 0), (1, 1), (1, 2),
         (2, 0), (2, 1), (2, 2)]
    """
    yield from self._comp.non_redundant_index_generator()