    @Test
    public void testnumPieces() {

        int N = 4;
        MutableBoard board = new MutableBoard(N);
        assertEquals(16, board.numPieces());
        board.set(0, 2, RED);
        board.set(1, 2, BLUE);
        board.set(2, 2, RED);
        board.set(3, 2, BLUE);
        assertEquals(20, board.numPieces());
    }