message: exit if no events are specified
diff: @@ -1060,7 +1060,11 @@ int main(int argc, char* argv[])
  {
  if (!group.empty()) ++nGroups;
  }
  -
  + if (nGroups == 0)
  + {
  + cout << "No events specified. Exiting.\n";
  + exit(EXIT_FAILURE);
  + }
  cout << "Collecting " << nGroups << " event groups\n";
  if (nGroups > 1)
  