static String structure(Object o) {

  new HashSet refd;

  return structure_2(structure_1(o, new structure_Data(refd)), refd);

}

// leave to false, unless unstructure() breaks

static boolean structure_allowShortening = false;

static int structure_shareStringsLongerThan = 20;

static class structure_Data {

  int stringSizeLimit;

  new IdentityHashMap<O, Integer> seen;

  HashSet<Integer> refd;

  new HashMap<S, Int> strings;

  new HashSet<S> concepts;

  Class conceptClass = findClass("Concept");

  *(HashSet<Int> *refd) {}

}

static String structure_1(Object o, structure_Data d) {

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

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

  if (o instanceof BigInteger)

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

  if (o instanceof Double)

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

  if (o instanceof Float)

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

  if (o instanceof Long)

    return o + "L";

  if (o instanceof Integer)

    return str(o);

  if (o instanceof Boolean)

    return ((Boolean) o).booleanValue() ? "t" : "f";

  if (o instanceof Character)

    ret quoteCharacter((Character) o);

  if (o instanceof File)

    ret "File " + quote(((File) o).getPath());

  // referencable objects follow

  Integer ref = d.seen.get(o);

  if (ref != null) {

    d.refd.add(ref);

    return "r" + ref;

  }

  if (o instanceof S && (ref = d.strings.get((S) o)) != null) {

    d.refd.add(ref);

    return "r" + ref;

  }

  ref = d.seen.size()+1;

  d.seen.put(o, ref);

  S r = "m" + ref + " "; // marker

  if (o instanceof S) {

    S s = d.stringSizeLimit != 0 ? shorten((S) o, d.stringSizeLimit) : (S) o;

    if (l(s) >= structure_shareStringsLongerThan)

      d.strings.put(s, ref);

    return r + quote(s);

  }

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

  StringBuilder buf = new StringBuilder();

  if (o instanceof HashSet)

    return r + "hashset " + structure_1(new ArrayList((Set) o), d);

  if (o instanceof TreeSet)

    return r + "treeset " + structure_1(new ArrayList((Set) o), d);

  if (o instanceof Collection && neq(name, "main$Concept$RefL")) {

    for (Object x : (Collection) o) {

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

      buf.append(structure_1(x, d));

    }

    return r + "[" + buf + "]";

  }

  if (o instanceof Map) {

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

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

      buf.append(structure_1(((Map.Entry) e).getKey(), d));

      buf.append("=");

      buf.append(structure_1(((Map.Entry) e).getValue(), d));

    }

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

  }

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

    if (o instanceof byte[])

      ret "ba " + quote(bytesToHex((byte[]) o));

    int n = Array.getLength(o);

    if (o instanceof bool[]) {

      S hex = boolArrayToHex((bool[]) o);

      int i = l(hex);

      while (i > 0 && hex.charAt(i-1) == '0' && hex.charAt(i-2) == '0') i -= 2;

      ret "boolarray " + n + " " + quote(substring(hex, 0, i));

    }

    S atype = "array", sep = ", ";

    if (o instanceof int[]) {

      //ret "intarray " + quote(intArrayToHex((int[]) o));

      atype = "intarray";

      sep = " ";

    }

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

      if (buf.length() != 0) buf.append(sep);

      buf.append(structure_1(Array.get(o, i), d));

    }

    return r + atype + "{" + buf + "}";

  }

  if (o instanceof Class)

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

  if (o instanceof Throwable)

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

  if (o instanceof BitSet) {

    BitSet bs = (BitSet) o;

    for (int i = bs.nextSetBit(0); i >= 0; i = bs.nextSetBit(i+1)) {

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

       buf.append(i);

    }

    return "bitset{" + buf + "}";

  }

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

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

    return r + String.valueOf(o);

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

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

    buf.append("l(" + structure_1(getOpt(o, "head"), d));

    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_1(arg, d));

      }

    buf.append(")");

    ret r + str(buf);

  }

  Class c = o.getClass();

  bool concept = d.conceptClass != null && d.conceptClass.isInstance(o);

  if (concept && !d.concepts.contains(c.getName())) {

    d.concepts.add(c.getName());

    r += "c ";

  }

  // serialize an object with fields.

  // first, collect all fields and values in fv.

  new TreeMap<S, O> fv;

  while (c != Object.class) {

    for (Field field : c.getDeclaredFields()) {

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

        continue;

      S fieldName = field.getName();

      Object value;

      try {

        field.setAccessible(true);

        value = field.get(o);

      } catch (Exception e) {

        value = "?";

      }

      // omit field "className" if equal to class's name

      if (concept && eq(fieldName, "className")

        && eq(value, shortName)) value = null;

      // put special cases here...

      if (value != null)

        fv.put(fieldName, value);

    }

    c = c.getSuperclass();

  }

  // Now we have fields & values. Process fieldValues if it's a DynamicObject.

  if (o instanceof DynamicObject) {

    fv.putAll((Map) fv.get("fieldValues"));

    fv.remove("fieldValues");

    shortName = (S) fv.get("className");

    fv.remove("className");

  }

  S singleField = fv.size() == 1 ? first(fv.keySet()) : null;

  // Render this$1 first because unstructure needs it for constructor call.

  if (fv.containsKey("this$1")) {

    buf.append("this$1=" + structure_1(fv.get("this$1"), d));

    fv.remove("this$1");

  }

  // Render the other fields.

  for (S fieldName : fv.keySet()) {

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

    buf.append(fieldName + "=" + structure_1(fv.get(fieldName), d));

  }

  String b = buf.toString();

  if (structure_allowShortening && singleField != null)

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

  String s = shortName;

  if (buf.length() != 0)

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

  return r + s;

}

// drop unused markers

static S structure_2(S s, HashSet<Integer> refd) {

  L<S> tok = javaTok(s);

  new StringBuilder out;

  for (int i = 1; i < l(tok); i += 2) {

    S t = tok.get(i);

    if (t.startsWith("m") && isInteger(t.substring(1))

      && !refd.contains(parseInt(t.substring(1))))

      continue;

    out.append(t).append(tok.get(i+1));

  }

  ret str(out);

}