anchor.diff:                      if q <= y then y, right
                       else find_del_right q right
   
  -                AvlDiet.join (x', y') left' right'
  +                AvlTree.Join comparer (x', y') left' right'
   
       /// Returns a new set with the given element removed.
       /// No exception is thrown if the set doesn't contain the specified element.
  
positive.code: 
      /// An empty DIET.
      let empty : CharDiet =
          AvlTree.empty
  
      /// Determines if a DIET is empty.
      let inline isEmpty (tree : CharDiet) =
          AvlTree.isEmpty tree
  
      /// Determines if a DIET contains a specified value.
      let rec contains value (tree : CharDiet) =
          match tree with
          | Empty ->
              false
          | Node ((x, y), left, right, _) ->
              if value < x then
                  contains value left
              elif value > y then
  
negative.code:         | Empty ->
              invalidArg "tree" "The tree is empty."
          | tree ->
              snd <| AvlTree.MaxElement tree
      
      /// Gets the minimum (least) value stored in the DIET.
      let minElement (tree : CharDiet) : char =
  