Remark Acc_lemma : forall y:B, Acc R y -> forall x:A, y = f x -> Acc Rof x.

Lemma Acc_simulation (Q : A -> A -> Prop) : forall b, Acc R b -> (forall a1 a2 b1, Q a2 a1 -> F a1 b1 -> exists b2, F a2 b2 /\ R b2 b1) -> forall a, F a b -> Acc Q a.