proposition: Lemma permK s : cancel s s^-1.
proof: Proof. by move=> x; rewrite -permM mulgV perm1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype tuple finfun bigop finset binomial) mathcomp(fingroup morphism)