public Builder addAllFixtures(
          java.lang.Iterable<? extends org.box2d.proto.Box2D.PbFixture> values) {
        if (fixturesBuilder_ == null) {
          ensureFixturesIsMutable();
          super.addAll(values, fixtures_);
          onChanged();
        } else {
          fixturesBuilder_.addAllMessages(values);
        }
        return this;
      }