// does not store null values
static <A> Map<S, A> indexFieldAsCIMap(Iterable<A> c, S field) {
  Map<S, A> map = ciMap();
  for (A a : unnull(c)) {
    S val = getString(a, field);
    if (val != null)
      map.put(val, a);
  }
  ret map;
}

static <A> Map<S, A> indexFieldAsCIMap(S field, Iterable<A> c) {
  ret indexFieldAsCIMap(c, field);
}