proposition: Lemma homo_mono_in : {in aD &, {homo f : x y / aR x y >-> rR x y}} -> {in rD &, {homo g : x y / rR x y >-> aR x y}} -> {in rD &, {mono g : x y / rR x y >-> aR x y}}.
proof: Proof. move=> mf mg x y hx hy; case: (boolP (rR _ _))=> [/mg //|]; first exact. by apply: contraNF=> /mf; rewrite !fgK ?mem_g//; apply. Qed.
imports: ssreflect ssrfun