F# tree printing

type tree<'a> (root : 'a, children : 'a tree list) =
    new (root, [<System.ParamArray>]children) = tree(root, List.ofArray children)
    member this.Root = root
    member this.Children = children

let rec lines
    format
    (t : 'a tree)
    : string seq
    =
    seq {
        yield sprintf format t.Root
        if t.Children.Length > 0 then
            let cLines spacer c = seq {
                let cLines = Array.ofSeq (lines format c)
                yield fst spacer + cLines.[0]
                for cl in Seq.skip 1 cLines -> snd spacer + cl
                }
            let cs = Array.ofList t.Children
            for i = 0 to t.Children.Length - 2 do
                yield! cLines ("├", "│") t.Children.[i]
            yield! cLines ("└", " ") t.Children.[t.Children.Length - 1]
    }

let t =
    tree (1,
        tree (2,
            tree 4,
            tree 6),
        tree (3,
            tree 6,
            tree 9),
        tree (4,
            tree 8))

for l in (lines "%A" t) do
    printfn "%s" l