code: public void readGroup(final int fieldNumber,
                          final MessageLite.Builder builder,
                          final ExtensionRegistryLite extensionRegistry)
        throws IOException {
      if (recursionDepth >= recursionLimit) {
        throw InvalidProtocolBufferException.recursionLimitExceeded();
      }
      ++recursionDepth;
      builder.mergeFrom(this, extensionRegistry);
      checkLastTagWas(
        WireFormat.makeTag(fieldNumber, WireFormat.WIRETYPE_END_GROUP));
      --recursionDepth;
    }
docs: Read a {@code group} field value from the stream.