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    #include "HepMC3/GenEvent.h"
    #include "HepMC3/ReaderAscii.h"
    #include "HepMC3/WriterAscii.h"
    #include "HepMC3/Print.h"
    
    #include <iostream>
    using namespace HepMC3;
    
    void neutrons_zdc(){
    
      //-------------------------------------
      WriterAscii hepmc_output("neutrons_zdc.hepmc");
      int        events_parsed = 0;
      GenEvent   evt(Units::GEV, Units::MM);
    
      for (events_parsed = 0; events_parsed < 100; events_parsed++) {
        // type 4 is beam 
        GenParticlePtr p1 =
            std::make_shared<GenParticle>(FourVector(0.0, 0.0, 10.0, 10.0), 11, 4);
        GenParticlePtr p2 = std::make_shared<GenParticle>(FourVector(0.0, 0.0, 0.0, 0.938), 2212, 4);
    
        double pz_part = 1.0 + events_parsed*1.0;
        // type 1 is final state 
        GenParticlePtr p3 =
            std::make_shared<GenParticle>(FourVector(0.0,0.0,pz_part, std::sqrt(pz_part*pz_part + 0.938*0.938) ), 2112, 1);
    
        GenVertexPtr v1 = std::make_shared<GenVertex>();
        v1->add_particle_in(p1);
        v1->add_particle_in(p2);
    
        v1->add_particle_out(p3);
        evt.add_vertex(v1);
    
        if (events_parsed == 0) {
          std::cout << "First event: " << std::endl;
          Print::listing(evt);
        }
    
        hepmc_output.write_event(evt);
        if (events_parsed % 10 == 0) {
          std::cout << "Event: " << events_parsed << std::endl;
        }
        evt.clear();
      }
      hepmc_output.close();
      std::cout << "Events parsed and written: " << events_parsed << std::endl;
    }