proposition: Lemma eq_inv_sym (w1 : seq S) (w2 : seq T) : eq_inv w1 w2 -> eq_inv w2 w1.
proof: Proof. rewrite /versions => Hinv i j; move/(_ i j) : Hinv; exact esym. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(perm fingroup) tools combclass ordtype permcomp