anchor.diff:  handle_call({register_client, Pid}, _From, State) ->
       NewClients = register_client(Pid, State#state.clients),
       {reply, ok, State#state{clients=NewClients}};
  +handle_call({deregister_client, Pid}, _From, State) ->
  +    NewClients = deregister_client(Pid, State#state.clients),
  +    {reply, ok, State#state{clients=NewClients}};
   handle_call(_Request, _From, State) ->
       {reply, ok, State}.
   
  +handle_cast({count_clients, Pid}, State) ->
  +    Count = count_clients(State#state.clients),
  +    Pid ! {client_count, Count},
  +    {noreply, State};
   handle_cast(_Msg, State) ->
       {noreply, State}.
   
  
positive.code: register_client(Pid) ->
      gen_server:call(?MODULE, {register_client, Pid}, infinity).
  
  clients() ->
      gen_server:call(?MODULE, clients, infinity).
  
  
negative.code: %% API
  -export([start_link/0,
           clients/0,
           register_client/1,
           deregister_client/1]).
  
  %% gen_server callbacks
  -export([init/1,
  