proposition: Lemma fproper1set A x : ([fset x] `<` A) -> (x \in A).
proof: Proof. by move/fproper_sub; rewrite fsub1set. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)