def METHOD_NAME(self):
    import ROOT as r
    self.prepareHistos()
    f = r.TFile("%s_%d.root" % (self.prefix_, 0), "RECREATE")
    num = 0
    for h in self.histos_:
        num = num + 1
        if num%1000 == 0:
            sys.stdout.write(".")
            sys.stdout.flush()
        h.book_and_fill(f)
    f.Write()
    f.Close()
    print('Wrote %d histograms in %d folders' % (len(self.histos_), len(self.folders_)))
    for i in range(1, self.numFiles_):
        subprocess.getoutput("cp %s_0.root %s_%d.root" % (self.prefix_,
                                                        self.prefix_,
                                                        i))
        print('x')