code: public void mergeFrom(final FieldSet<FieldDescriptorType> other) {
      for (int i = 0; i < other.fields.getNumArrayEntries(); i++) {
        mergeFromField(other.fields.getArrayEntryAt(i));
      }
      for (final Map.Entry<FieldDescriptorType, Object> entry :
               other.fields.getOverflowEntries()) {
        mergeFromField(entry);
      }
    }
docs: Like {@link Message.Builder#mergeFrom(Message)}, but merges from another
  {@link FieldSet}.