proposition: Lemma permE f f_inj : @perm T f f_inj =1 f.
proof: Proof. by move=> x; rewrite -pvalE [@perm]unlock ffunE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype tuple finfun bigop finset binomial) mathcomp(fingroup morphism)