// 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;
}

Began life as a copy of #1001005