proposition: Lemma fset_notin_add y X x : y ∉ {{ x }} ⊔ X ↔ y ≠ x ∧ y ∉ X.
proof: Proof. rewrite fset_in_add. tauto. 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