@Override
              public Integer apply(Integer from) {
                return from - 1;
              }
            });
    assertMapsEqual(expected, map);
  }

  public void testTransformEntrySetContains() {
    Map<@Nullable String, @Nullable Boolean> underlying = new HashMap<>();
    underlying.put("a", null);
    underlying.put("b", true);
    underlying.put(null, true);