bf front_camera  % front_camera
bf back_camera  % back_camera
bf touch_screen  % touch_screen
bf basic_screen  % basic_screen
bf keypad  % keypad
bf calling_feature  % calling_feature
bf mp3  % mp3
bf gps  % gps
bf battery  % battery
camera = front_camera + back_camera + front_camera . back_camera % camera
screen = touch_screen + basic_screen % screen
acessories = gps + camera + gps . camera + mp3 + gps . mp3 + camera . mp3 + gps . camera . mp3 % acessories
batteries = battery + battery . battery % batteries
mobile = screen . calling_feature . ( keypad + 1) . ( acessories + 1) . batteries % mobile
