boost::mutex::scoped_lock lm (_mutex);
shared_ptr<dcp::InteropSubtitleAsset> iop = dynamic_pointer_cast<dcp::InteropSubtitleAsset> (sc);
boost::mutex::scoped_lock lm (_mutex);
shared_ptr<dcp::InteropSubtitleAsset> iop = dynamic_pointer_cast<dcp::InteropSubtitleAsset> (sc);