anchor.diff:  register_client(Pid) ->
       gen_server:call(?MODULE, {register_client, Pid}, infinity).
   
  +deregister_client(Pid) ->
  +    gen_server:call(?MODULE, {deregister_client, Pid}, infinity).
  +
   clients() ->
       gen_server:call(?MODULE, clients, infinity).
   
  
positive.code: %% API
  -export([start_link/0,
           clients/0,
           register_client/1]).
  
  %% gen_server callbacks
  -export([init/1,
  
negative.code: 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) ->
  