// optimistic signature that usually holds

static <A> A clone(A o) {

  new ArrayDeque<Runnable> q;

  Object x = clone_impl(o, new IdentityHashMap, q);

  while (!q.isEmpty())

    q.poll().run();

  return (A) x;

}

static O clone_impl(O o, final IdentityHashMap seen, final ArrayDeque<Runnable> q) ctex {

  if (o == null) null;

  Object y = seen.get(o);

  if (y != null) return y;

  if (o instanceof L) {

    //print("Cloning list: " + o);

    final new L l;

    seen.put(o, l);

    for (final O x : (L) o)

      q.add(r { l.add(clone_impl(x, seen, q)) });

    return l;

  }

  if (o instanceof Map) {

    final Map m = similarEmptyMap((Map) o);

    seen.put(o, m);

    for (O entry : ((Map) o).entrySet()) {

      final Map.Entry e = (Map.Entry) entry;

      q.add(r {

        m.put(clone_impl(e.getKey(), seen, q), clone_impl(e.getValue(), seen, q))

      });

    }

    ret m;

  }

  if (o instanceof S || o instanceof Number || o instanceof Bool) ret o;

  if (o instanceof O[]) {

    final O[] l = (O[]) o;

    final O[] l2 = l.clone();

    seen.put(o, l2);

    for (int i = 0; i < l.length; i++) {

      final int _i = i;

      q.add(r { l2[_i] = clone_impl(l[_i], seen, q) });

    }

    ret l2;

  }

  // clone an arbitrary custom object

  //print("Cloning custom: " + o);

  final O clone = nuObjectWithoutArguments(o.getClass());

  seen.put(o, clone);

  Class c = o.getClass();

  while (c != O.class) {

    Field[] fields = c.getDeclaredFields();

    for (final Field field : fields) {

      if ((field.getModifiers() & Modifier.STATIC) != 0)

        continue;

      field.setAccessible(true);

      final O value = field.get(o);

      q.add(r { field.set(clone, clone_impl(value, seen, q)) });

    }

    c = c.getSuperclass();

  }

  ret clone;

}