type ProperNum =
    | ProperNum
    static member instance (ProperNum, ProperNum, _) = fun () -> ()
    static member instance (ProperNum, _ : Zero, _) = fun () -> Zero
    static member inline instance (ProperNum, _ : Succ<'a>, _) = fun () -> Succ(Inline.instance(ProperNum, Unchecked.defaultof<'a>) ())

let inline instance() : 'a = Inline.instance(ProperNum, Unchecked.defaultof<'a>) ()