TString tt(Int_t len = 10){
  gSystem->Sleep(1500);
  srand (time(NULL));
  TString str = ""; 
  static const char alphanum[] =
    "0123456789"
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    "abcdefghijklmnopqrstuvwxyz";

  for (int i = 0; i < len; ++i) {
    str += alphanum[rand() % (sizeof(alphanum) - 1)];
  }
 
  return str;
}