public Builder addFixtures(
          int index, org.box2d.proto.Box2D.PbFixture.Builder builderForValue) {
        if (fixturesBuilder_ == null) {
          ensureFixturesIsMutable();
          fixtures_.add(index, builderForValue.build());
          onChanged();
        } else {
          fixturesBuilder_.addMessage(index, builderForValue.build());
        }
        return this;
      }