anchor.diff:  %% Adds data to the tree.
   add(Data) ->
       gen_server:call(?SERVER, {add, Data}).
  +
  +quit() ->
  +    gen_server:call(?SERVER, quit).
   
   addNValues(0) ->
       ok;
  
positive.code: -behaviour(gen_server).
  
  %% API
  -export([start_link/0, walk/0, add/1, addNValues/1, addValues/1, testforn/1]).
  
  %% gen_server callbacks
  -export([init/1, handle_call/3, handle_cast/2, handle_info/2,
  
negative.code:         end,
      F(State#state.left),
      F(State#state.right),
      {stop, normal, ok, State}.
  
  %%--------------------------------------------------------------------
  %% @private
  