English
If x ≃ y, then for any w, w ∈ x ↔ w ∈ y.
Русский
Если x эквивалентно y, тогда для любого w верно w ∈ x ⇔ w ∈ y.
LaTeX
$$$\\\\forall {x y : PSet},\\\\ x\\\\simeq y \\\\rightarrow \\\\forall {w : PSet}, w\\\\in x \\\\leftrightarrow w\\\\in y$$$
Lean4
theorem congr_right : ∀ {x y : PSet.{u}}, Equiv x y → ∀ {w : PSet.{u}}, w ∈ x ↔ w ∈ y
| ⟨_, _⟩, ⟨_, _⟩, ⟨αβ, βα⟩, _ =>
⟨fun ⟨a, ha⟩ =>
let ⟨b, hb⟩ := αβ a
⟨b, ha.trans hb⟩,
fun ⟨b, hb⟩ =>
let ⟨a, ha⟩ := βα b
⟨a, hb.euc ha⟩⟩