proposition: Lemma opt_absorbs_left A (R E : rel A) : E @ R << R -> opt E @ opt R << opt R.
proof: Proof. intros ER x z [y [xy yz]]. inversion yz; clear yz; subst. inversion xy; clear xy; subst. apply opt_intro. apply ER. exists x0. fo. Qed.
imports: Coq(Setoid Basics Morphisms List Lia) CoLoR(LogicUtil)