anchor.diff:                  let d = dist x y
                   cardinal_aux (acc + d + 1) (left :: right :: ts)
           
  -        cardinal_aux 0 [t]
  +        cardinal_aux 0 [tree]
   
       /// Returns the number of intervals in the set.
  -    let intervalCount (t : CharDiet) =
  +    let intervalCount (tree : CharDiet) =
  +        // Preconditions
  +        assert (AvlTree.AvlInvariant tree)
  +
           // OPTIMIZE : Modify this to use a mutable stack instead of an F# list.
           let rec cardinal_aux acc = function
               | [] -> acc
  
positive.code: 
      //
      let rec internal (*private*) find_del_left p (tree : CharDiet) : char * CharDiet =
          match tree with
          | Empty ->
              p, Empty
          | Node (left, right, (x, y), _) ->
              if p > succ y then
                  let p', right' = find_del_left p right
                  p', AvlTree.Join (comparer, left, right', (x, y))
              elif p < x then
                  find_del_left p left
  
negative.code:                     if b <= y then y, right
                      else find_del_right b right
  
                  assert (AvlTree.AvlInvariant left')
                  assert (AvlTree.AvlInvariant right')
                  AvlTree.Join (comparer, left', right', (x', y'))
  
      /// Returns a new set with the given element removed.
      /// No exception is thrown if the set doesn't contain the specified element.
      let rec remove value (tree : CharDiet) : CharDiet =
          // Preconditions
          assert (AvlTree.AvlInvariant tree)
  
          match tree with
          | Empty ->
              Empty
  