anchor.diff:  handle_call(_Request, _From, State) ->
       {reply, ignored, State}.
   
  +handle_cast(stop, State) ->
  +    SessionList = get_all_sessions(),
  +    delete_sessions(SessionList),
  +    {stop, normal, State};
   handle_cast(_Msg, State) ->
       {noreply, State}.
   
  
positive.code: start_link() ->
      gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
  
  
  -spec new_session() -> {ok, pid()}.
  new_session() ->
  
negative.code: 
  %% API.
  -export([start_link/0]).
  -export([stop/0]).
  
  -export([new_session/0]).
  -export([get_session/1]).
  