'Return a set of basis elements.
>>> from sympy.abc import x
>>> from sympy import QQ
>>> QQ.old_poly_ring(x).free_module(3).basis()
([1, 0, 0], [0, 1, 0], [0, 0, 1])'

def basis(self):

    from sympy.matrices import eye
    M = eye(self.rank)
    return tuple((self.convert(M.row(i)) for i in range(self.rank)))
