anchor.diff:        % We then tell the user to send all future messages to its receiver
         %   instead of us.
         Sender ! {Receiver, logged_in},
  -      % Finally, we register the receiver to the user's name in our state and proceed.
  -      master_actor(Subscriptions, gb_trees:insert(UserName, Receiver, Receivers), Channels);
  +      % Finally, we proceed.
  +      master_actor(Subscriptions, Channels);
   
       {Sender, log_out, UserName} ->
         % We first notify all channels the user subscribes to and dispose of the receiver.
         log_out(Sender, dict:fetch(UserName, Subscriptions), Channels),
         % We then notify the sender that we successfully logged the user out.
         Sender ! {self(), logged_out},
  -      % Finally we proceed with the user removed from our receivers tree.
  -      master_actor(Subscriptions, gb_trees:delete(UserName, Receivers), Channels);
  +      % Finally, we proceed.
  +      master_actor(Subscriptions, Channels);
   
       {Sender, Receiver, join_channel, UserName, ChannelName} ->
         % We first subscribe the user to the channel.
  
positive.code:       % We do not forget to notify the sender that the user has successfully joined the channel.
        Sender ! {self(), channel_joined},
        % Finally, we proceed with the new state.
        master_actor(NewSubscriptions, Receivers, dict:store(ChannelName, ChannelPid, Channels));
  
      {Sender, get_channel_history, ChannelName} ->
        % We forward requests for channel histories to the channel processes.
        dict:fetch(ChannelName, Channels) ! {Sender, get_channel_history},
        % We then proceed with the same state.
        master_actor(Subscriptions, Receivers, Channels)
    end.
  
  -spec log_in(UserPid  :: pid(),
  
negative.code:       %   not logged in at this point.
        Sender ! {self(), user_registered},
        % Finally, we proceed with the new state.
        master_actor(NewSubscriptions, Channels);
  
      {Sender, log_in, UserName} ->
        % We first create a receiver and notify all channels the user subscribes to.
  