// "seen" is now default

static String structure(Object o) {

  return structure(o, 0, new IdentityHashMap);

}

static String structure_seen(Object o) {

  return structure(o, 0, new IdentityHashMap);

}

// leave to false, unless unstructure() breaks

static boolean structure_allowShortening = false;

static String structure(Object o, int stringSizeLimit, IdentityHashMap<O, Integer> seen) {

  if (o == null) return "null";

  // these are never back-referenced (for readability)

  if (o instanceof String)

    return quote(stringSizeLimit != 0 ? shorten((String) o, stringSizeLimit) : (String) o);

  if (o instanceof BigInteger)

    return "bigint(" + o + ")";

  if (o instanceof Double)

    return "d(" + quote(str(o)) + ")";

  if (o instanceof Long)

    return o + "L";

  if (o instanceof Integer)

    return str(o);

  if (seen != null) {

    Integer ref = seen.get(o);

    if (ref != null)

      return "r" + ref;

    seen.put(o, seen.size()+1);

  }

  String name = o.getClass().getName();

  StringBuilder buf = new StringBuilder();

  if (o instanceof HashSet)

    return "hashset" + structure(new ArrayList((Set) o), stringSizeLimit, seen);

  if (o instanceof TreeSet)

    return "treeset" + structure(new ArrayList((Set) o), stringSizeLimit, seen);

  if (o instanceof Collection) {

    for (Object x : (Collection) o) {

      if (buf.length() != 0) buf.append(", ");

      buf.append(structure(x, stringSizeLimit, seen));

    }

    return "[" + buf + "]";

  }

  if (o instanceof Map) {

    for (Object e : ((Map) o).entrySet()) {

      if (buf.length() != 0) buf.append(", ");

      buf.append(structure(((Map.Entry) e).getKey(), stringSizeLimit, seen));

      buf.append("=");

      buf.append(structure(((Map.Entry) e).getValue(), stringSizeLimit, seen));

    }

    return (o instanceof HashMap ? "hashmap" : "") + "{" + buf + "}";

  }

  if (o.getClass().isArray()) {

    int n = Array.getLength(o);

    for (int i = 0; i < n; i++) {

      if (buf.length() != 0) buf.append(", ");

      buf.append(structure(Array.get(o, i), stringSizeLimit, seen));

    }

    return "array{" + buf + "}";

  }

  if (o instanceof Class)

    return "class(" + quote(((Class) o).getName()) + ")";

  if (o instanceof Throwable)

    return "exception(" + quote(((Throwable) o).getMessage()) + ")";

  // Need more cases? This should cover all library classes...

  if (name.startsWith("java.") || name.startsWith("javax."))

    return String.valueOf(o);

  String shortName = o.getClass().getName().replaceAll("^main\\$", "");

  if (shortName.equals("Lisp")) {

    buf.append("l(" + structure(getOpt(o, "head"), stringSizeLimit, seen));

    L args = cast getOpt(o, "args");

    if (nempty(args))

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

        buf.append(", ");

        O arg = args.get(i);

        // sweet shortening

        if (arg != null && eq(arg.getClass().getName(), "main$Lisp") && isTrue(call(arg, "isEmpty")))

          arg = get(arg, "head");

        buf.append(structure(arg, stringSizeLimit, seen));

      }

    buf.append(")");

    ret str(buf);

  }

  int numFields = 0;

  String fieldName = "";

  if (shortName.equals("DynamicObject")) {

    shortName = (String) get(o, "className");

    Map<String, Object> fieldValues = (Map) get(o, "fieldValues");

    for (String _fieldName : fieldValues.keySet()) {

      fieldName = _fieldName;

      Object value = fieldValues.get(fieldName);

      if (value != null) {

        if (buf.length() != 0) buf.append(", ");

        buf.append(fieldName + "=" + structure(value, stringSizeLimit, seen));

      }

      ++numFields;

   }

  } else {

    // regular class

    Class c = o.getClass();

    while (c != Object.class) {

      Field[] fields = c.getDeclaredFields();

      for (Field field : fields) {

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

          continue;

        fieldName = field.getName();

        // skip outer object reference

        if (fieldName.indexOf("$") >= 0) continue;

        Object value;

        try {

          field.setAccessible(true);

          value = field.get(o);

        } catch (Exception e) {

          value = "?";

        }

        // put special cases here...

        if (value != null) {

          if (buf.length() != 0) buf.append(", ");

          buf.append(fieldName + "=" + structure(value, stringSizeLimit, seen));

        }

        ++numFields;

      }

      c = c.getSuperclass();

    }

  }

  String b = buf.toString();

  if (numFields == 1 && structure_allowShortening)

    b = b.replaceAll("^" + fieldName + "=", ""); // drop field name if only one

  String s = shortName;

  if (buf.length() != 0)

    s += "(" + b + ")";

  return s;

}