'Scale the function by a term independent of x.
f(x) -> s * f(x)
This is fast, if Fourier series of f(x) is already
computed.
Examples
>>> from sympy import fourier_series, pi
>>> from sympy.abc import x
>>> s = fourier_series(x**2, (x, -pi, pi))
>>> s.scale(2).truncate()
-8*cos(x) + 2*cos(2*x) + 2*pi**2/3'

def scale(self, s):

    (s, x) = (sympify(s), self.x)
    if (x in s.free_symbols):
        raise ValueError(("'%s'    should    be    independent    of    %s" % (s, x)))
    an = self.an.coeff_mul(s)
    bn = self.bn.coeff_mul(s)
    a0 = (self.a0 * s)
    sfunc = (self.args[0] * s)
    return self.func(sfunc, self.args[1], (a0, an, bn))
