HepMC::ShortEventAttribute Node1 HepMC::ShortEventAttribute + NUP + IDPRUP + XWGTUP + SCALUP + AQEDUP + AQCDUP + IDUP + ISTUP + MOTHUP + ICOLUP + PUP + VTIMUP + SPINUP + ShortEventAttribute() + ShortEventAttribute() + from_event() + from_string() + to_string() + resize() Node2 HepMC3::Attribute     Node2->Node1