anchor.diff:  %% API
   -export([start_link/0,
            login/0,
  -         post/1]).
  +         post/1,
  +         is_logged_in/0
  +        ]).
   
   %% gen_server callbacks
   -export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]).
  
positive.code: 
  post(Text) ->
      gen_server:call(?MODULE, {post, Text}).
  
  %%%===================================================================
  %%% gen_server callbacks
  
negative.code: handle_call({post, Text}, _From, State) ->
      Status = post(Text, State),
      {reply, Status, State};
  handle_call({is_logged_in}, _From, State) ->
      {reply, State#state.logged_in, State};
  handle_call(_Request, _From, State) ->
      {reply, unknown_command, State}.
  
  