anchor.diff:  
     [<MethodImpl(MethodImplOptions.AggressiveInlining)>]
     let rec add (comparer: KeyComparer<'K>) k v (m:MapTree<'K,'b>) = // was 'V instead of 'K, slightly measleading
  -      if isNull m then
  +      if isEmpty m then
           MapTree(k,v)
         else
           match m with
  
positive.code:         | _ -> acc + 1
      sizeAux 0 x
  
    let empty : MapTree<_,_> = null 
  
    let inline height (m:MapTree<'a,'b>) =
      if isNull m then 0
      else
        match m with
        | :? MapTreeNode<_,_> as mn -> mn.Height
        | _ -> 1
  
    let inline isEmpty m = isNull m
  
    let inline mk l k v r = 
        if isNull l && isNull r then
            MapTree(k,v)
        else 
            let hl = height l 
  
negative.code: 
  [<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
  module internal MapTree = 
    let inline isEmpty (m:MapTree<_,_>) = isNull m
  
    let size x =
      let rec sizeAux acc (m:MapTree<'a,'b>) =
        if isEmpty m then
          acc
        else
          match m with
  