変換後のコンピュテーション式を覗いてみたところ、規則には出てこないはずのLetがでてきたので一応それを残しておく。

translated_computation.fs

open System
open Quotations.Patterns

type Maybe<'a> = Nothing | Just of 'a

type MaybeBuilder () =
  member __.Quote() = ()
  member __.Run(x: Quotations.Expr<'a>) =
    Console.WriteLine(x)
    Just 42

  member __.Zero() = Nothing
  member __.Return(x) = Just x
  member __.ReturnFrom(x) = x
  member __.Bind(x, f) = match x with Just v -> f v | _ -> Nothing
  member __.Using(x: #IDisposable, f) =
    try
      f x
    finally
      match x with
      | null -> ()
      | _ -> x.Dispose()

let maybe = MaybeBuilder()

let returnJust x = Just x
let returnNothing _ = Nothing

[<EntryPoint>]
let main argv = 
  maybe {
    let! j = returnJust 10
    return j
  }
  |> ignore

  Console.ReadKey false |> ignore
  0