proposition: Lemma tcastE m n (eq_mn : m = n) t i : tnth (tcast eq_mn t) i = tnth t (cast_ord (esym eq_mn) i).
proof: Proof. by case: n / eq_mn in i *; rewrite cast_ord_id. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq choice fintype path)