Fixpoint lst_length (l : term_list) : nat :=
  match l with
  | tnil => 0
  | tcons _ l' => S (lst_length l')
  end.Fixpoint term_high (t : term) : nat :=
  match t with
  | app a l => S (term_high_0 l)
  end
 
 with term_high_0 (l : term_list) : nat :=
  match l with
  | tnil => 0
  | tcons hd tl => max (term_high hd) (term_high_0 tl)
  end.

Lemma high_aux_1 : forall (a : ad) (l : term_list), S (term_high_0 l) = term_high (app a l).