Counter-intuitive behaviour of the let-in construct

let-in.fs

let working () =
  let a = 6 in
    begin // or (
      printfn "%A" a
      let a = 5 in
        printfn "%A" a
    end // or )
  let b = a in
    printfn "%A" b

let ugly () =
  let a = 6 in
      printfn "%A" a ; // <- the semicolon is needed, the indentation level does not matter (as long as it is more indented than the let)
    let a = 5 in
      printfn "%A" a
  let b = a in
    printfn "%A" b

let notcompiling () =
  let a = 6 in
    printfn "%A" a
    let a = 5 in
      printfn "%A" a
  let b = a in
    printfn "%A" b