proposition: Lemma Sphi_p3 x i j: ordinalp x -> i <o j -> Sphi i (Sphi j x) = Sphi j x.
proof: Proof. move: (all_der_pow_aux) => [ha hb] ox ij. exact: (all_der_p7 ha hb ox ij). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)