static Object clone(Object o) {
  return clone(o, new IdentityHashMap);
}

static Object clone(Object o, IdentityHashMap seen) ctex {
  if (o == null)
    return o;
    
  Object y = seen.get(o);
  if (y != null) return y;
  
  if (o instanceof List) {
    print("Cloning list: " + o);
    List l = new ArrayList();
    seen.put(o, l);
    for (Object x : (List) o)
      l.add(clone(x, seen));
    return l;
  }
  
  if (o instanceof Map) {
    Map m = o instanceof TreeMap ? new TreeMap() : new HashMap();
    seen.put(o, m);
    for (O entry : ((Map) o).entrySet()) {
      Map.Entry e = (Map.Entry) entry;
      m.put(clone(e.getKey(), seen), clone(e.getValue(), seen));
    }
    return m;
  }
  
  if (o instanceof String || o instanceof Number || o instanceof Boolean)
    return o;
    
  if (o instanceof Object[]) {
    Object[] l = (Object[]) o;
    Object[] l2 = l.clone();
    seen.put(o, l2);
    for (int i = 0; i < l.length; i++)
      l2[i] = clone(l[i], seen);
    return l2;
  }
  
  // clone an arbitrary custom object

  // TODO: use Constructor object and setAccessible
  print("Cloning custom: " + o);
  Object clone = o.getClass().newInstance();
  seen.put(o, clone);
  Class c = o.getClass();
  while (c != Object.class) {
    Field[] fields = c.getDeclaredFields();
    for (Field field : fields) {
      if ((field.getModifiers() & Modifier.STATIC) != 0)
        continue;
      Object value;
      field.setAccessible(true);
      value = field.get(o);
      field.set(clone, clone(value, seen));
    }
    c = c.getSuperclass();
  }
  return clone;
}