English
A symmetry variant: the same mem-class statement holds with the roles swapped, using commutation of the Setoid classes.
Русский
Симметрическое утверждение для класса эквивалентности с использованием коммутирования классов.
LaTeX
$$Eqv class membership is preserved under commutation: eqv_class_mem' asserts the same inclusion with swapped arguments.$$
Lean4
theorem eqv_class_mem' {c : Set (Set α)} (H : ∀ a, ∃! b ∈ c, a ∈ b) {x} : {y : α | mkClasses c H x y} ∈ c :=
by
convert @Setoid.eqv_class_mem _ _ H x using 3
rw [Setoid.comm']