modula-2

For university we're supposed to write a game (the language is modula2 little chance you know it). Each tetromino is defined through a central piece and 3 relative pieces, Now when it comes to rotation i just rotate the relative blocks (linear algebra). What i am asking myself is how to handle the rotations a user does when the piece has already "landed" because than the user has a little time left to move his pieve quickly into the desired position but in tetris versions all over the web you can "rotate over" other pieces, but i dont seem to get it with my rotation. Over what piece do i need