'Return a sequence of elements `args` such that cls(*args) == expr
>>> from sympy import Symbol, Mul, Add
>>> x, y = map(Symbol, \'xy\')
>>> Mul.make_args(x*y)
(x, y)
>>> Add.make_args(x*y)
(x*y,)
>>> set(Add.make_args(x*y + y)) == set([y, x*y])
True'

@classmethod
def make_args(cls, expr):

    if isinstance(expr, cls):
        return expr.args
    else:
        return (sympify(expr),)
