proposition: Lemma fset_notin_singleton_neq x y : x ∉ {{ y }} ↔ x ≠ y.
proof: Proof. now rewrite fset_in_singleton_eq. Qed.
imports: MathClasses.theory.lattices MathClasses.varieties.monoids MathClasses.implementations.bool MathClasses.implementations.list_finite_set MathClasses.orders.lattices MathClasses.interfaces.abstract_algebra MathClasses.interfaces.finite_sets MathClasses.interfaces.orders