// does not store null values
static SmallestListMultiMap smallestListMultiMapIndexByMethod(Collection c, S method, O... args) {
  new SmallestListMultiMap map;
  for (O a : c) {
    O val = callOpt(a, method, args);
    if (val != null)
      map.put(val, a);
  }
  ret map;
}