private boolean overlaps(Piece pi, int x0, int y0) {
		if( pi.w() + x0 > w() ){
			return true;
		}
		if( pi.h() + y0 > h() ){
			return true;
		}
		
		for( int x = 0 ; x < pi.w() ; x++ ){
			for( int y = 0 ; y < pi.h() ; y++ ){
				if( pi.contains(x, y) ){
					if( _board[x+x0][y+y0] != null ){
						return true;
					}
				}
			}
		}
		return false;
	}