anchor.diff:  
   -spec largest(b_tree()) -> key_value().
   
  -largest({_, _, _, 0, nil} = BTree) ->
  +largest(?B_TREE_EMPTY = BTree) ->
       erlang:error({empty_tree, BTree});
   largest({_, _, _, _, Tree}) ->
       largest_1(Tree).
  
positive.code: 
  -spec keys(b_tree()) -> keys().
  
  keys({_, _, _, 0, nil}) ->
      [];
  keys({_, _, _, _, {_, _, KeyValues, Trees}}) ->
      keys(KeyValues, Trees, []).
  
negative.code: 
  -spec height(b_tree()) -> non_neg_integer().
  
  height(?B_TREE_EMPTY) ->
      0;
  height({_, _, _, _, Tree}) ->
      height(Tree, 1).
  