proposition: Lemma homoRL_in : {in aD &, {homo f : x y / aR x y >-> rR x y}} -> {in rD & aD, forall x y, aR (g x) y -> rR x (f y)}.
proof: Proof. by move=> Hf x y hx hy /Hf; rewrite fgK ?mem_g// ?inE; apply. Qed.
imports: ssreflect ssrfun