Section MAIN.
Variable A : Setoid.
Variable n m : Nat.
Variable v : seq n A.
Variable w : seq m A.
(** - More facts about concatenation *)

Lemma concat_first_part :
forall (i : Nat) (Hi : i < n) (Hi' : i < n + m),
(v ++ w) (Build_finiteT Hi') =' v (Build_finiteT Hi) in _.