Notation "{ 'perm' T }" := (perm_of (Phant T))
(at level 0, format "{ 'perm'  T }") : type_scope.
Arguments pval _ _%g.
Bind Scope group_scope with perm_type.
Bind Scope group_scope with perm_of.
Notation "''S_' n" := {perm 'I_n}
(at level 8, n at level 2, format "''S_' n").
Local Notation fun_of_perm_def := (fun T (u : perm_type T) => val u : T -> T).
Local Notation perm_def := (fun T f injf => Perm (@perm_proof T f injf)).
Module Type PermDefSig.
Parameter fun_of_perm : forall T, perm_type T -> T -> T.
Parameter perm : forall (T : finType) (f : T -> T), injective f -> {perm T}.

Axiom fun_of_permE : fun_of_perm = fun_of_perm_def.