proposition: Lemma diff_id_sh : 1 != sh.
proof: Proof. by apply/eqP; move/(congr1 (fun p : {perm square} => p c0)); rewrite !permE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype tuple finfun bigop finset fingroup) mathcomp(action perm primitive_action)