'Truncated series.
Returns truncated series expansion of f upto
order ``O(x**n)``.
If n is ``None``, returns an infinite iterator.'

def truncate(self, n=6):

    if (n is None):
        return iter(self)
    (x, x0) = (self.x, self.x0)
    pt_xk = self.xk.coeff(n)
    if (x0 is S.NegativeInfinity):
        x0 = S.Infinity
    return (self.polynomial(n) + Order(pt_xk, (x, x0)))
