public abstract int[][] toNestedIndexes();
@Test
    void testToNestedIndexes() {
        assertThat(Projection.of(new int[] {1, 2, 3, 4}).toNestedIndexes())
                .isEqualTo(
                        new int[][] {new int[] {1}, new int[] {2}, new int[] {3}, new int[] {4}});
        assertThat(
                        Projection.of(new int[][] {new int[] {4}, new int[] {1, 3}, new int[] {2}})
                                .toNestedIndexes())
                .isEqualTo(new int[][] {new int[] {4}, new int[] {1, 3}, new int[] {2}});
    }