proposition: Lemma ttail_etrace et : ttail (etrace et) = etail et.
proof: Proof. rewrite /etail /etrace /etrace_perm /ttail proper_permt. case: (even_trace et); first by rewrite !permt_id. by case: et => [|[] ?] //=; rewrite -2![permt _ _]map_comp; apply: eq_map; case. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype)