Definition rgd_fun (p : {perm T'}) :=
[fun x1 => if insub x1 is Some u then sval (p u) else x].

(* Specifications: *)
Lemma rgdP p : injective (rgd_fun p).
Definition rgd p := perm (@rgdP p).
Lemma rfd_odd (p : {perm T}) : p x = x -> rfd p = p :> bool.
Lemma rfd_iso : 'C_('Alt_T)[x | 'P] \isog 'Alt_T'.
End Restrict.
Lemma simple_Alt5 (T : finType) : #|T| >= 5 -> simple 'Alt_T.