void mergetrees(std::string f1, std::string f2, std::string output) {
    
 // Load files 
 TFile file1(f1.c_str());
 TFile file2(f2.c_str());
 // Load trees
 TTree* tree1 = (TTree*)file1.Get("tracks");
 TTree* tree2 = (TTree*)file2.Get("tracks"); 
 //if (tree1->GetEntries() != tree2->GetEntries()) ;
 // Setup output file
 TFile _output(output.c_str(), "RECREATE");
 // Setup new trees
 //_versionTree = new TTree("version", "version");
 //_euhits = new TTree("fitpoints", "fitpoints");
 //_zstree = new TTree("rawdata", "rawdata");
 TTree* _eutracks = new TTree("tracks", "tracks");
 //
 int _nTrackParams;
 std::vector<double> *_xPos;
 std::vector<double> *_yPos;
 std::vector<double> *_dxdz;
 std::vector<double> *_dydz;
 std::vector<int> *_trackIden;
 std::vector<int> *_trackNum;
 std::vector<double> *_chi2;
 std::vector<double> *_ndof;
 // Setup branches
 _eutracks->SetAutoSave(1000000000);
 _eutracks->Branch("xPos", &_xPos);

 for (int i = 0; i < tree1->GetEntries(); ++i) {
  tree1->GetEntry(i);
  std::cout << i << std::endl;
  auto vec = tree1->GetLeaf("xPos")->GetValue(0);
  std::cout << vec << std::endl;
 }

}