svoid mapMethodLike internByteArrayField(S field, Cl l) {
  new HashMap<HashedByteArray, byte[]> map;
  for (O o : l) {
    byte[] value = cast getOpt(o, field);
    if (value == null) continue;
    byte[] existing = map.get(value);
    if (existing != null)
      setOpt(o, field, value);
    else
      map.put(HashedByteArray(value), value);
  }
}