English
For Encodable α, and any n,a, a ∈ decode₂ α n iff a ∈ decode n and encode a = n.
Русский
Для Encodable α и любых n, a: a входит в decode₂ α n тогда и только тогда, когда a ∈ decode n и encode a = n.
LaTeX
$$$a \in \operatorname{decode}_{2}(\alpha,n) \iff a \in \operatorname{decode}(n) \land \operatorname{encode}(a) = n$$$
Lean4
instance _root_.PUnit.encodable : Encodable PUnit :=
⟨fun _ => 0, fun n => Nat.casesOn n (some PUnit.unit) fun _ => none, fun _ => by simp⟩