static String structure(Object o) {

  return structure(o, 0);

}

// leave to false, unless unstructure() breaks

static boolean structure_allowShortening = false;

static String structure(Object o, int stringSizeLimit) {

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

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

  StringBuilder buf = new StringBuilder();

  if (o instanceof HashSet)

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

  if (o instanceof TreeSet)

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

  if (o instanceof Collection) {

    for (Object x : (Collection) o) {

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

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

    }

    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));

      buf.append("=");

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

    }

    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));

    }

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

  }

  if (o instanceof String)

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

  if (o instanceof Class)

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

  if (o instanceof Throwable)

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

  if (o instanceof BigInteger)

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

  if (o instanceof Double)

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

  if (o instanceof Long)

    return o + "L";

  // 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")));

    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));

      }

    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));

      }

      ++numFields;

   }

  } else {

    // regular class

    // TODO: go to superclasses too

    Field[] fields = o.getClass().getDeclaredFields();

    for (Field field : fields) {

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

        continue;

      Object value;

      try {

        field.setAccessible(true);

        value = field.get(o);

      } catch (Exception e) {

        value = "?";

      }

      fieldName = field.getName();

      // put special cases here...

      if (value != null) {

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

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

      }

      ++numFields;

    }

  }

  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;

}