Hint Constructors PPO.

Lemma same_rank_same_ppo vv ff cc (t : term vv ff cc) s rk rk':
(forall f, In f (functions_of_term t) -> rk f = rk' f) ->
(forall f, In f (functions_of_term s) -> rk f = rk' f) ->
PPO rk t s -> PPO rk' t s.