anchor.diff:  
   /// AVL Tree.
   [<NoEquality; NoComparison>]
  +[<StructuredFormatDisplay("{DisplayFormatted}")>]
   [<DebuggerTypeProxy(typedefof<AvlTreeDebuggerProxy<int>>)>]
  -[<CompilationRepresentation(CompilationRepresentationFlags.UseNullAsTrueValue)>]
  +//[<CompilationRepresentation(CompilationRepresentationFlags.UseNullAsTrueValue)>]
   type internal AvlTree<'T when 'T : comparison> =
       /// Empty tree.
       | Empty
  
positive.code:         // Preconditions
          //assert (AvlTree.AvlInvariant l)
          //assert (AvlTree.AvlInvariant r)
  
          match l, r with
          | Empty, Empty ->
  
negative.code:         // Preconditions
          //assert (AvlTree.AvlInvariant l)
          //assert (AvlTree.AvlInvariant r)
          // TODO : Assert all values in 'l' are less than all values in 'r'.
  
          match l, r with
          | Empty, Empty ->
  