protected type Barrier is
  procedure Arrive;
private
  entry Go;             -- used to delay early arrivals
  count : Integer := 0; -- number who have arrived
  time_to_leave : Boolean := False;
end Barrier;

protected body Barrier is
  procedure Arrive is begin
    count := count+1;
    if count < N then
      requeue Go;   -- wait for others to arrive
    else
      count := count-1; time_to_leave := True;
    end if;
  end;

  entry Go when time_to_leave is begin
    count := count-1;
    if count = 0 then time_to_leave := False; end if;
  end;
end Barrier;