code: public static <A, B> FeatureGenerator<A, B> combinedFeatureGenerator(
        Iterable<FeatureGenerator<A, B>> generators) {
      return new CombinedFeatureGenerator<A, B>(generators);
    }
docs: Combines many {@code FeatureGenerator}s into a single
  {@code FeatureGenerator}. The returned generator returns the union of all
  features generated by {@code generators}. If multiple generators output the
  same feature, the returned generator sums their counts.