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