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

        assertThat(
                        Projection.of(new int[][] {new int[] {4}, new int[] {1}, new int[] {2}})
                                .toTopLevelIndexes())
                .isEqualTo(new int[] {4, 1, 2});

        assertThatThrownBy(
                        () ->
                                Projection.of(
                                                new int[][] {
                                                    new int[] {4}, new int[] {1, 3}, new int[] {2}
                                                })
                                        .toTopLevelIndexes())
                .isInstanceOf(IllegalStateException.class);
    }