from sympy.polys.rings import ring
    >>> from sympy.polys.ring_series import rs_trunc
    >>> R, x = ring('x', QQ)
    >>> p = x**10 + x**5 + x + 1
    >>> rs_trunc(p, x, 12)
    x**10 + x**5 + x + 1
    >>> rs_trunc(p, x, 10)
    x**5 + x + 1
    N)