Fixpoint first_activations_rec (proof_tree : cbv) : list cbv :=
  match proof_tree with
  | cbv_constr lp _ _ _ _ => flat_map first_activations_rec lp
  | cbv_split lp p _ _ _ _ => first_activations_rec p ++ flat_map first_activations_rec lp
  | cbv_update _ _ _ _ _ _ _ as p' => [p']
  | cbv_read _ _ _ => []
  end.Definition first_activations (proof_tree : cbv) : list cbv :=
  match proof_tree with
  | cbv_constr lp _ _ _ _ => []
  | cbv_split lp p _ _ _ _ => []
  | cbv_update _ _ p _ _ _ _ => first_activations_rec p
  | cbv_read _ _ _ => []
  end.Definition strict_activations (proof_tree: cbv) :=
  flat_map activations (first_activations proof_tree).

Lemma first_activation_is_update proof_tree p : In p (first_activations proof_tree) -> exists i s p' c1 t c2 v, p = cbv_update i s p' c1 t c2 v.