anchor.diff:  is_defined(_, {_, _, 0, _, _, nil}) ->
       false;
   is_defined(Key, {_, _, _, SortFunction, State, Tree}) ->
  -    case lookup_1(Key, Tree, SortFunction, State) == none of
  -        true ->
  +    case lookup_1(Key, Tree, SortFunction, State) of
  +        none ->
               false;
           _ ->
               true
  
positive.code:         none ->
              % Look ahead.
              {SubtreeKeyNo, _, SubtreeKeyValues, _} = Subtree = lists:nth(SubtreePos, Subtrees),
              case SubtreeKeyNo == KeyNoMax of
                  % Split node.
                  true ->
                      {SplitKeyValues, SplitSubtrees1, SplitTree1, SplitTree2, SplitSubtrees2} = split_node_non_root(KeyNo, KeyValues, Subtrees, Subtree, SubtreePos, SubtreeNoMin, SortFunction, nil),
                      {
                          KeyNo + 1,
  
negative.code: % Leaf node.
  lookup_1(Key, {KeyNo, 0, KeyValues, []}, SortFunction, _) ->
      {Value, _} = binary_search(Key, KeyValues, KeyNo, 1, KeyNo, SortFunction),
      case Value of
          none ->
              Value;
          _ ->
              {value, Value}
  