type Direction = Up | Down

​

type BoxTree<'Item> =

  | Item        of 'Item

  | BoxSet      of BoxTree<'Item> list * Direction

let rec (|Split|_|) fnCheck tree =  

  match tree with

  | BoxSet (a::b::[],dir) when fnCheck a -> 

      Split (a,b,dir) |> Some

  | BoxSet (_,_)  

  | Item _  -> 

      None

​

let tryFindSplitOnItem checkFn tree =

  match tree with

  | Split checkFn box -> Some(box)

  | _  -> None