anchor.diff:  register_client(Pid, Clients) ->
       sets:add_element(Pid, Clients).
   
  +%% @spec dwregister_client(pid(), set()) -> set()
  +%% @doc Deregister a new client.
  +deregister_client(Pid, Clients) ->
  +    sets:del_element(Pid, Clients).
  +
   %% @spec count_clients(set()) -> number()
   %% @doc Count clients.
   count_clients(Clients) ->
  
positive.code: register_client(Pid) ->
      gen_server:call(?MODULE, {register_client, Pid}, infinity).
  
  clients() ->
      gen_server:call(?MODULE, clients, infinity).
  
  
negative.code: %% API
  -export([start_link/0,
           clients/0,
           register_client/1,
           deregister_client/1]).
  
  %% gen_server callbacks
  -export([init/1,
  