anchor.diff:  		already_visited -> {reply, false, Tree};
   		{updated, NewTree} -> {reply, true, NewTree}
   	end.
  +
  +terminate(normal, _) -> ok.
  +
  +
  +%% Internal functions
   
   check_url([], {true, _}) -> already_visited;
   check_url([], {false, Tree}) -> {updated, {true, Tree}};
  
positive.code: 				{updated, NewTree} -> {updated, {NodeStatus, gb_trees:update(Part, NewTree, Parent)}}
  			end
  	end.
  
  terminate(normal, _) -> ok.
  
negative.code: -export([init/1, handle_call/3, terminate/2]). %% gen_server callbacks
  -behavior(gen_server).
  
  
  %% External API
  
  start_link() -> gen_server:start_link(?MODULE, [], []).
  
  book_visit(Pid, URL) -> gen_server:call(Pid, {book, URL}).
  
  
  %% Callbacks for gen_server
  
  init([]) -> {ok, {true, gb_trees:empty()}}.
  
  