Abstract for group Tetris

Bricks composed of four squares drop from the top of screen
sequentially.  The user rotates the bricks and moves them to left or
to right.  A brick stops when touches the bottom of the screen or any
brick piled up from the bottom of the screen.

The game ends when the bricks pile touch the top of the screen.  A row
disappears when bricks fill a horizontal line, and the pile drops
down, and more room is emptied out.