static Set<S> combinedMechSetsCI(Collection<S> listNames) {
  if (empty(listNames)) ret ciSet();
  if (l(listNames) == 1) ret mechSetCI(first(listNames));
  Set<S> set = ciSet();
  for (S listName : listNames)
    addIfNempty(set, mL(listName));
  ret set;
}