proposition: Lemma cards2 a b : #|[set a; b]| = (a != b).+1.
proof: Proof. by rewrite -card2; apply: eq_card=> x; rewrite !inE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)