anchor.diff:              AvlTree.mkt_bal_r (new_n, l, right)
   
       /// Determines if a AvlTree contains a specified value.
  +    [<Pure>]
       static member Contains (comparer : IComparer<'T>, tree : AvlTree<'T>, value : 'T) =
           match tree with
           | Empty ->
  
positive.code:     let rec private union' (input : CharDiet) limit head (stream : CharDiet)
          : CharDiet * (char * char) option * CharDiet =
          // Preconditions
          assert (AvlTree.Height input >= AvlTree.Height stream)
          assert (dietInvariant input)
          assert (dietInvariant stream)
  
  
negative.code: 
          match tree with
          | Empty ->
              CharDiet.Singleton (a, b)
              |> tapInvariant
          | Node (left, right, (x, y), _) ->
              if b < pred x then
                  let left' = addRange (a, b) left
                  assert (dietInvariant left')
                  CharDiet.Join (comparer, left', right, (x, y))
                  |> tapInvariant
              elif a > succ y then
                  let right' = addRange (a, b) right
                  assert (dietInvariant right')
                  CharDiet.Join (comparer, left, right', (x, y))
                  |> tapInvariant
              else
                  // Now, we know the interval (a, b) being inserted either overlaps or is
                  // adjancent to the current inverval (x, y), so we merge them.
  