Ltac hypInv :=
      try (match goal with
          | [H: Some _ = Some _ |-_ ] => inversion H; clear H; subst
      end); try pairInv.Fixpoint lookupIndex (A : Set)(eqd : eq_dec A)(ls : list A)(a : A) def :=
  match ls with
    | nil => def
    | a' :: ls' =>
      if (eqd a a') then O else S (lookupIndex eqd ls' a def)
  end.

Lemma skipn_app : forall (A : Set)(ls1 ls2 : list A), skipn (length ls1) (ls1 ++ ls2) = ls2.