anchor.diff:      // additions to fit f#+ :
       let inline map f (x:DList<_>)    = DList.foldBack (cons << f ) x empty
       let concat x = DList.fold append empty x 
  -    let inline join (f:DList<DList<_>>) = concat f
  -    let inline ap f x = join <| map (fun y -> map ((|>) y) f) x
  +    let inline ap f x = concat <| map (fun y -> map ((|>) y) f) x
       let inline bind m k              = DList.foldBack (append << k) empty m
   
   type DList<'T> with
  
positive.code: /// DList is an ordered linear structure implementing the List signature (head, tail, cons), 
  /// end-insertion (add), and O(1) append. Ordering is by insertion history.
  /// DList is an implementation of [John Hughes' append list](http://dl.acm.org/citation.cfm?id=8475).
  type DList<'T>(length : int , data : DListData<'T> ) =
      let mutable hashCode = None
      member internal this.dc = data
  
negative.code:     static member Return x = DList (1, x)
      static member Map (x, f) = DList.map f x
      static member (<*>) (f, x) = DList.ap f x
      static member (>>=) (x, f) = DList.bind x f
  