anchor.diff:      /// Applies the given accumulating function to all elements in a DIET.
       let foldBack (folder : char -> 'State -> 'State) (tree : CharDiet) (state : 'State) =
           // Preconditions
  -        checkNonNull "tree" tree
  +        // NONE -- Skip null check because the Empty tree is represented as null.
   
           let folder = FSharpFunc<_,_,_>.Adapt folder
   
  
positive.code:     /// Applies the given accumulating function to all elements in a DIET.
      let fold (folder : 'State -> char -> 'State) (state : 'State) (tree : CharDiet) =
          // Preconditions
          checkNonNull "tree" tree
  
          let folder = FSharpFunc<_,_,_>.Adapt folder
  
  
negative.code:     static let empty = CharSet (Diet.empty)
  
      //
      static member Empty
          with get () = empty
      
  