Ropes in F#

// Learn more about F# at http://fsharp.net
#light
module Module1

open System

type Rope = Rope of int * int * RopeNode
and RopeNode = 
 Concat of Rope * Rope
 | Leaf of String
// | Lazy of (Unit -> Rope)

let depth = fun (Rope(depth,length,content)) -> depth
let length = fun (Rope(depth,length,content)) -> length

let concat a b = Rope(max (depth a) (depth b),
                      length a + length b,
                      Concat(a,b))

let rec subrope start offset rope = 
    let subrope_ x = match x with
        | Concat(rope1, rope2) -> 
            let left = 
                if (start <= 0 && offset >= length rope1) then
                    rope1
                else subrope start offset rope
            let right =
                if (start <= length rope1 && start + offset >= length rope1 + length rope2) then 
                    rope2
                else subrope (start - length rope1) (offset - length left) rope
            concat left right 
        | Leaf(str) -> 
            let substring = str.Substring(start, offset)
            Rope(1,offset,Leaf(substring))
    match rope with Rope(d, l, node) -> subrope_ node

let rec fromString (str : String) = 
    if str.Length < 20 then
        Rope(1, str.Length, Leaf(str))
    else
        let left = str.Substring(0, str.Length/2)
        let right =  str.Substring(str.Length/2)
        concat (fromString left) (fromString right)

let rec toStrings = function 
        | Rope(d,l,Leaf(str)) -> [str]
        | Rope(d,l,Concat(r1, r2)) -> append (toStrings r1)  (toStrings r2)

type RopeIterator = class
    interface Enumerable with
        member x.