type Tree<'a> = 

  | Empty

  | Node of 'a * Tree<'a> * Tree<'a>

​

let traverse (tree : Tree<'a>) =

  let rec loop (trees : seq<Tree<'a>>) = seq {

    let values = 

      trees

      |> Seq.choose (function

        | Empty -> None

        | Node (value, _, _) -> Some value)

​

    yield! values

​

    let subtrees = 

      trees 

      |> Seq.collect (function 

        | Empty -> Seq.empty

        | Node (value, left, right) -> seq { yield left; yield right })

      |> Seq.toArray

​

    if subtrees.Length > 0 then

      yield! loop subtrees

  }

​

  loop <| seq { yield tree }

​

let tree = 

  Node (1, 

    Node (2, 

      Node (4, Empty, Empty), 

      Node (5, Empty, Empty)), 

    Node (3, 

      Node (6, Empty, Empty), 

      Empty))

​

traverse tree |> Seq.iter (printfn "%d")