type System.Collections.Generic.List<'T> with
  member this.RemoveAtUnordered(index:int) =
    this.[index] <- this.[this.Count-1]
    this.RemoveAt(this.Count-1)
  member this.RemoveUnordered(item:'T) =
    let index = this.IndexOf(item)
    if index <> - 1 then this.RemoveAtUnordered(index)