// does not store null values
static MultiMap treeMultiMapIndexByField(Collection c, S field) {
  MultiMap map = treeMultiMap();
  for (O a : c) {
    O val = getOpt(a, field);
    if (val != null)
      map.put(val, a);
  }
  ret map;
}

static MultiMap mapMethodLike treeMultiMapIndexByField(S field, Collection c) {
  ret treeMultiMapIndexByField(c, field);
}