anchor.diff:  
     [<MethodImpl(MethodImplOptions.AggressiveInlining)>]
     let rec find (comparer: KeyComparer<'K>) k (m:MapTree<'K,'b>) =
  -    if isNull m then
  +    if isEmpty m then
         raise (System.Collections.Generic.KeyNotFoundException())
       else
         match m with
  
positive.code: 
  [<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
  module internal MapTree = 
    let size x = 
      let rec sizeAux acc (m:MapTree<'a,'b>) =
        if isNull m then
          acc
        else
          match m with
  
negative.code: 
    [<MethodImpl(MethodImplOptions.AggressiveInlining)>]
    let rec add (comparer: KeyComparer<'K>) k v (m:MapTree<'K,'b>) = // was 'V instead of 'K, slightly measleading
        if isEmpty m then
          MapTree(k,v)
        else
          match m with
  