void anaMeritSvacNtuple(const char* ra, const char* rb, TNtuple* tuple, TH1F** h) {

  // Author: Chen, Xin (xchen@slac.stanford.edu)
  // Additions and comments by anders :-) 

  // Input Root file (SVAC ntuple):
  TFile f(ra);

  // 'Output' is the name of the Root tree in the SVAC ntuple.
  TTree* t1 = (TTree*) f.Get("Output");

  // Get the Merit tree:
  TFile fMerit(rb);
  TTree* t2 = (TTree*) fMerit.Get("MeritTuple");

  //
  // Declare variables to hold data read from the tree.
  // Note some variables in the SVAC ntuple are arrays, so it is neccessary
  // to have an array to hold the data loaded from the SVAC ntuple file.
  // Check the description of the SVAC ntuple on the Instrument Analysis 
  // web page  to determine size and dimension of the array
  int nStrips[16][18][2];
  int nTkrNumDigis;
  float dir[3];

  // ALL MERIT VARIABLES ARE 'double'!
  double nbrTracks;

  //
  // Load the branches we want. The branch name is the same as the 
  // variable name in the SVAC ntuple. You can find a complete list 
  // of SVAC variables in the description available from the main 
  // Instrument Analysis Page.
  //

  // i.e. here we load the SVAC variable 'TkrNumStrips'.
  TBranch* brTkrNumStrips = t1->GetBranch("TkrNumStrips");
  brTkrNumStrips->SetAddress(&nStrips);

  TBranch* brTkrNumDigis = t1->GetBranch("TkrNumDigis");
  brTkrNumDigis->SetAddress(&nTkrNumDigis);

  TBranch* brVtxXDir = t1->GetBranch("VtxXDir");
  brVtxXDir->SetAddress(&dir[0]);

  TBranch* brVtxYDir = t1->GetBranch("VtxYDir");
  brVtxYDir->SetAddress(&dir[1]);

  TBranch* brVtxZDir = t1->GetBranch("VtxZDir");
  brVtxZDir->SetAddress(&dir[2]);

  // Load the Merit variable 'TkrNumTracks':
  TBranch* brTkrNumTracks = t2->GetBranch("TkrNumTracks");
  brTkrNumTracks->SetAddress(&nbrTracks);


  // Get the total number of events:
  //int nEvt = (int) t1->GetEntries();
  int nEvt = (int) t2->GetEntries();

  // For testing:
  //nEvt = 100;

  // Some output .....
  cout << "nEvent = " << nEvt << endl;


  // Loop over all events:
  for(int i = 0; i != nEvt; ++i) {
    
    // Get the events:
    t1->GetEntry(i);
    t2->GetEntry(i);

    // Fill output ntuple with SVAC and Merit variables:
    tuple->Fill(nTkrNumDigis, nbrTracks, dir[0], dir[1], dir[2]);

    // Cut: We only want to look at the events with less than 6 digis!
    if(nTkrNumDigis >= 6) continue;

    // Loop over tracker bilayers and views:
    for(int biLayer = 0; biLayer != 18; ++biLayer) {
      for(int view = 0; view != 2; ++view) {
	// Hits in this plane?
	if(nStrips[0][biLayer][view] > 0) {
	  // Fill histogram with planes that are hit:
	  h[0]->Fill(getPlane(biLayer, view));
	}
      }
    }
  }
}

// 
// Given the biLayer number and the view, return the 
// corresponding plane number. Plane 0 is at the bottom.
//
int getPlane(int biLayer, int view)
{
  static int map[18][2];
  static bool first = true;
  if(first) {
    map[0][1] = 0;
    map[0][0] = 1;
    map[1][0] = 2;
    map[1][1] = 3;
    map[2][1] = 4;
    map[2][0] = 5;
    map[3][0] = 6;
    map[3][1] = 7;
    map[4][1] = 8;
    map[4][0] = 9;
    map[5][0] = 10;
    map[5][1] = 11;
    map[6][1] = 12;
    map[6][0] = 13;
    map[7][0] = 14;
    map[7][1] = 15;
    map[8][1] = 16;
    map[8][0] = 17;
    map[9][0] = 18;
    map[9][1] = 19;
    map[10][1] = 20;
    map[10][0] = 21;
    map[11][0] = 22;
    map[11][1] = 23;
    map[12][1] = 24;
    map[12][0] = 25;
    map[13][0] = 26;
    map[13][1] = 27;
    map[14][1] = 28;
    map[14][0] = 29;
    map[15][0] = 30;
    map[15][1] = 31;
    map[16][1] = 32;
    map[16][0] = 33;
    map[17][0] = 34;
    map[17][1] = 35;
    first = false;
  }

  return map[biLayer][view];
}