proposition: Lemma cardfs2 a b : #|` [fset a; b]| = (a != b).+1.
proof: Proof. by rewrite !cardfsU1 cardfs1 inE addn1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)