int eta_write_ascii () 
{
  PReaction* R = new PReaction ("3.5", "p", "p", "p p eta");
  
  // Defining variables further used to write to ASCII file

  R->Do ("eta_px = [eta]->Px() ; eta_py = [eta]->Py() ;"
         "eta_pz = [eta]->Pz() ; eta_m  = [eta]->M () ") ;

  // Writing our variables to ASCII output file

  R->Output("eta_sample.txt", "echo $eta_px $eta_py $eta_pz $eta_m");
  
  R->Print();
  R->Loop (1000) ;
  return 0;
}