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 obj.ReferenceEquals(m, null) then
  +      if isNull m then
           MapTree(k,v)
         else
           match m with
  
positive.code: 
    [<MethodImpl(MethodImplOptions.AggressiveInlining)>]
    let rec find (comparer: KeyComparer<'K>) k (m:MapTree<'K,'b>) =
      if obj.ReferenceEquals(m, null) then
        raise (System.Collections.Generic.KeyNotFoundException())
      else
        match m with
  
negative.code: module internal MapTree = 
    let size x = 
      let rec sizeAux acc (m:MapTree<'a,'b>) =
        if isNull m then
          acc
        else
          match m with
  