proposition: Lemma Image_inj_sub A B (f : A -> B) (X1 X2 : Pred A) : injective f -> Image f X1 <=p Image f X2 -> X1 <=p X2.
proof: Proof. by move=>H E x /(Image_mem f) /E [y] /H ->. Qed.
imports: Coq(ssreflect ssrbool ssrfun Setoid) mathcomp(ssrnat seq eqtype) pcm(options)