proposition: Lemma subCset_P A B E: sub A E -> sub B E -> (sub (E -s A) B <-> sub (E -s B) A).
proof: Proof. move => ae be; rewrite -{2} (setC_K ae) -{1} (setC_K be). apply: subsetC_P; apply: sub_setC. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)