proposition: Lemma cardfs0 : #|` @fset0 K| = 0.
proof: Proof. by rewrite -(@card_fsub fset0) // fsub0 cards0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)