int TTree_AddFriend() {
  TFile* fin = new TFile ("simple_branch.root");
  TTree* t1 = fin->Get<TTree> ("tree_time"  ) ,
       * t2 = fin->Get<TTree> ("tree_energy") ;
  t1->AddFriend (t2) ;
  
  t1->Draw ("time:energy");
  return 0;
}