Definition vector_firstPart_h(A : Set)(m1 m2 : nat)(v : Vector.t A (m1 + m2)) : {ls : list A | length ls = m1}.Fixpoint firstPart(A : Set)(m1 m2 : nat)(v : Vector.t A (m1 + m2)) : Vector.t A m1 :=
  of_sig_list (vector_firstPart_h m1 m2 v).

Theorem skipn_length : forall (A : Set) n0 (ls : list A), length (skipn n0 ls) = (length ls) - n0.