proposition: Lemma cardfsID B A : #|` A `&` B| + #|` A `\` B| = #|` A|.
proof: Proof. by rewrite -!(@card_fsub A) // fsubI fsubD cardsID. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)