'Shift the function by a term independent of x.
f(x) -> f(x) + s
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.shift(1).truncate()
-4*cos(x) + cos(2*x) + 1 + pi**2/3'

def shift(self, s):

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