Never instantiated, piece is the generic superclass for 7 specific pieces. Piece factors out the commonalities among the Tetris pieces-- each piece is an array of 4 squares, each piece has a center of rotation, each piece can only move if it first asks its squares if they can move, each piece can only add itself to the board if its squares say "no, we can't move."

subclasses: boxshape/lshapea/lshapeb/tshape/stick/doublelinea/doublelineb
reference: PieceFactory