...
    (x0, x + y)
    (x1, (x0 - 1)**2)
    (x2, 2*x + 1)
    (_3, x0/x2 + x1)
    (_4, x2**x0)
    (x2, None)
    (_0, x1)
    (x1, None)
    (_2, x0)
    (x0, None)
    (_1, x)
    >>> print(rvs)
    (_0, _1, _2, _3, _4)
    z