English
Equivalence of propositions is the same as iff.
Русский
Эквивалентность propositions равна Iff.
LaTeX
$$$\text{ofIff}: (P \leftrightarrow Q) \Rightarrow P \simeq Q$$$
Lean4
/-- `ULift α` is equivalent to `α`. -/
@[simps (attr := grind =) -fullyApplied apply symm_apply]
protected def ulift {α : Type v} : ULift.{u} α ≃ α :=
⟨ULift.down, ULift.up, ULift.up_down, ULift.down_up.{v, u}⟩