proposition: Lemma fset2P x a b : reflect (x = a \/ x = b) (x \in [fset a; b]).
proof: Proof. by rewrite !inE; apply: (iffP orP) => [] [] /eqP ->; [left|right|left|right]. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)