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