proposition: Lemma cardsD A B : #|A :\: B| = (#|A| - #|A :&: B|)%N.
proof: Proof. by rewrite -(cardsID B A) addKn. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)