@Test public void testToImmutableTable_covariance() {

  ImmutableTable<Object, Object, Iterable<?>> table =
      BiStream.of("ab", 1, "bc", 2)
          .collect(groupingBy((s, i) -> s.charAt(0), groupingBy(s -> s.charAt(1), toList())))
          .collect(toImmutableTable());
  assertThat(table.rowMap())
      .containsExactly('a', ImmutableMap.of('b', asList(1)), 'b', ImmutableMap.of('c', asList(2)))
      .inOrder();
}