'Substitute `x + a` for `x`.'

def shift_x(self, a):

    x = self.x
    listaftershift = self.annihilator.listofpoly
    base = self.annihilator.parent.base
    sol = [base.from_sympy(base.to_sympy(i).subs(x, (x + a))) for i in listaftershift]
    sol = DifferentialOperator(sol, self.annihilator.parent)
    x0 = (self.x0 - a)
    if (not self._have_init_cond()):
        return HolonomicFunction(sol, x)
    return HolonomicFunction(sol, x, x0, self.y0)
