anchor.diff:  %%% handled in some other place.
   %%%
   %%% Among other things process state contains:
  -%%% - current set of room with their descriptions
  +%%% - current set of rooms with their descriptions
   %%% - information about connectivity between rooms
   %%% - set of commands currently available to user
   %%%
  
positive.code: start_link() ->
      gen_server:start_link(?MODULE, [], []).
  
  %% @doc Sends an user command to game server and awaits for reply that
  %% should be presented to the user.
  -spec command(Cmd :: binary(), Args :: [binary()], GenServerRef :: term()) -> term().
  command(Cmd, Args, ServerRef) ->
  
negative.code:                            [string:join(GroupsAsStrings, ", ")],
                             Reply)}.
  
  %% @doc Stops the game
  -spec cmd_quit(Cmd :: binary(), Args :: [binary()], State :: state(), Reply :: reply()) -> {state(), reply()}.
  cmd_quit(_Cmd, _Object, State, Reply) ->
      {State#{status := quit}, Reply}.
  