anchor.diff:      /// If the tree already contains the value, no exception is thrown;
       /// the tree will be returned without modification.
       static member Insert (comparer : IComparer<'T>, tree : AvlTree<'T>, value : 'T) =
  +        // Preconditions
  +        //assert (AvlTree.AvlInvariant tree)
  +
           match tree with
           | Empty ->
               Node (Empty, Empty, value, 1u)
  
positive.code:             else
                  let root, r' = AvlTree.DeleteMin r
                  AvlTree.Join (comparer, l, r', root)
  
  //
  and [<Sealed>]
  
negative.code:     /// Removes the maximum (greatest) value from an AvlTree,
      /// returning the value along with the updated tree.
      static member DeleteMax (tree : AvlTree<'T>) =
          // Preconditions
          assert (AvlTree.AvlInvariant tree)
  
          match tree with
          | Empty ->
              invalidArg "tree" "Cannot delete the maximum value from an empty tree."
  