'Changes only the variable of Holonomic Function, for internal
purposes. For composition use HolonomicFunction.composition()'

def change_x(self, z):

    dom = self.annihilator.parent.base.dom
    R = dom.old_poly_ring(z)
    (parent, _) = DifferentialOperators(R, 'Dx')
    sol = []
    for j in self.annihilator.listofpoly:
        sol.append(R(j.rep))
    sol = DifferentialOperator(sol, parent)
    return HolonomicFunction(sol, z, self.x0, self.y0)
