void make_files(int start)
{
  Int_t x=start,y=start+1,z=start+2;
  Int_t xeven, yeven, zeven;
  
  TFile* f1 = new TFile(Form("data%d.root",start),"RECREATE");
  TTree* data = new TTree("data","data");
  data->Branch("x",&x);
  data->Branch("y",&y);
  data->Branch("z",&z);
  for(int i=0; i<5; ++i){
    data->Fill();
    x=z+1; y=x+1; z=y+1;
  }
  data->Write();
  
  TFile* f2 = new TFile(Form("xeven%d.root",start),"RECREATE");
  TTree* xetree = new TTree("cuts","cuts");
  xetree->Branch("xeven",&xeven);
  
  TFile* f3 = new TFile(Form("yeven%d.root",start),"RECREATE");
  TTree* yetree = new TTree("cuts","cuts");
  yetree->Branch("yeven",&yeven);
  
  TFile* f4 = new TFile(Form("zeven%d.root",start),"RECREATE");
  TTree* zetree = new TTree("cuts","cuts");
  zetree->Branch("zeven",&zeven);
  
  for(int i=0; i<data->GetEntries(); ++i){
    data->GetEntry(i);
    xeven = (x%2==0);
    yeven = (y%2==0);
    zeven = (z%2==0);
    xetree->Fill();
    yetree->Fill();
    zetree->Fill();
  }
  
  f2->cd();
  xetree->Write();
  f3->cd();
  yetree->Write();
  f4->cd();
  zetree->Write();
  
  delete f1;
  delete f2;
  delete f3;
  delete f4;
}

TChain* chainfriends()
{
  make_files(1);
  make_files(6);
  make_files(11);
  make_files(15);
  
  TChain* cut1 = new TChain("xeven");
  TChain* cut2 = new TChain("yeven");
  TChain* cut3 = new TChain("zeven");
  cut1->AddFriend(cut2);
  cut1->AddFriend(cut3);
  
  cut1->Add("xeven*.root/cuts");
  cut2->Add("yeven*.root/cuts");
  cut3->Add("zeven*.root/cuts");
  
  cout<<"xeven: "<<cut1->GetEntries("xeven")
      <<" yeven: "<<cut2->GetEntries("yeven")
      <<" zeven: "<<cut3->GetEntries("zeven")
      <<endl;
  
  cout<<"xeven: "<<cut1->GetEntries("xeven")
      <<" yeven: "<<cut1->GetEntries("yeven")
      <<" zeven: "<<cut1->GetEntries("zeven")
      <<endl;
  
  return cut1;
 
}