static String structureConcept(Object o) {
  new HashSet refd;
  new StringBuilder out;
  structureConcept_1(out, o, new structureConcept_Data(refd));
  return structureConcept_2(str(out), refd);
}

// leave to false, unless unstructure() breaks
static boolean structureConcept_allowShortening = false;

static int structureConcept_shareStringsLongerThan = 20;

static class structureConcept_Data {
  int stringSizeLimit;
  new IdentityHashMap<O, Integer> seen;
  HashSet<Integer> refd;
  new HashMap<S, Int> strings;
  new HashSet<S> concepts;
  Class conceptClass = findClass("Concept");
  bool markConcepts;
  new Map<S, Concept> unlisted; // unlisted empty instances to check default field values
  
  *(HashSet<Int> *refd) {}
}

static void structureConcept_1(StringBuilder out, Object o, structureConcept_Data d) {
  if (o == null) { out.append("null"); ret; }
  
  // these are never back-referenced (for readability)
  
  if (o instanceof BigInteger) {
    out.append("bigint(").append(o).append(")"); ret;
  }
  
  if (o instanceof Double) {
    out.append("d(").append(quote(str(o))).append(")"); ret;
  }
    
  if (o instanceof Float) {
    out.append("fl ").append(quote(str(o))); ret;
  }
    
  if (o instanceof Long) {
    out.append(o).append("L"); ret;
  }
  
  if (o instanceof Integer) {
    out.append(str(o)); ret;
  }
    
  if (o instanceof Boolean) {
    out.append(((Boolean) o).booleanValue() ? "t" : "f"); ret;
  }
    
  if (o instanceof Character) {
    out.append(quoteCharacter((Character) o)); ret;
  }
    
  if (o instanceof File) {
    out.append("File ").append(quote(((File) o).getPath())); ret;
  }
    
  // referencable objects follow
  
  Integer ref = d.seen.get(o);
  if (ref != null) {
    d.refd.add(ref);
    out.append("r").append(ref); ret;
  }
      
  if (o instanceof S && (ref = d.strings.get((S) o)) != null) {
    d.refd.add(ref);
    out.append("r").append(ref); ret;
  }
  
  ref = d.seen.size()+1;
  d.seen.put(o, ref);
  out.append("m").append(ref).append(" "); // marker

  if (o instanceof S) {
    S s = d.stringSizeLimit != 0 ? shorten((S) o, d.stringSizeLimit) : (S) o;
    if (l(s) >= structureConcept_shareStringsLongerThan)
      d.strings.put(s, ref);
    out.append(quote(s)); ret;
  }
    
  String name = o.getClass().getName();

  if (o instanceof HashSet) {
    out.append("hashset ");
    structureConcept_1(out, new ArrayList((Set) o), d);
    ret;
  }

  if (o instanceof TreeSet) {
    out.append("treeset ");
    structureConcept_1(out, new ArrayList((Set) o), d);
    ret;
  }
  
  if (o instanceof Collection && neq(name, "main$Concept$RefL")) {
    if (!o instanceof ArrayList)
      out.append(shortDynamicClassName(o));
    out.append("[");
    int l = out.length();
    for (Object x : (Collection) o) {
      if (out.length() != l) out.append(", ");
      structureConcept_1(out, deref(x), d);
    }
    out.append("]");
    ret;
  }
  
  if (o instanceof Map) {
    if (o instanceof HashMap) out.append("hm");
    out.append("{");
    int l = out.length();
    for (Object e : ((Map) o).entrySet()) {
      if (out.length() != l) out.append(", ");
      structureConcept_1(out, ((Map.Entry) e).getKey(), d);
      out.append("=");
      structureConcept_1(out, ((Map.Entry) e).getValue(), d);
    }
    out.append("}");
    ret;
  }
  
  if (o.getClass().isArray()) {
    if (o instanceof byte[]) {
      out.append("ba ").append(quote(bytesToHex((byte[]) o))); ret;
    }

    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;
      out.append("boolarray ").append(n).append(" ").append(quote(substring(hex, 0, i))); ret;
    }
    
    S atype = "array", sep = ", ";

    if (o instanceof int[]) {
      //ret "intarray " + quote(intArrayToHex((int[]) o));
      atype = "intarray";
      sep = " ";
    }
    
    out.append(atype).append("{");
    for (int i = 0; i < n; i++) {
      if (i != 0) out.append(sep);
      structureConcept_1(out, Array.get(o, i), d);
    }
    out.append("}"); ret;
  }

  if (o instanceof Class) {
    out.append("class(").append(quote(((Class) o).getName())).append(")"); ret;
  }
    
  if (o instanceof Throwable) {
    out.append("exception(").append(quote(((Throwable) o).getMessage())).append(")"); ret;
  }
    
  if (o instanceof BitSet) {
    BitSet bs = (BitSet) o;
    out.append("bitset{");
    int l = out.length();
    for (int i = bs.nextSetBit(0); i >= 0; i = bs.nextSetBit(i+1)) {
      if (out.length() != l) out.append(", ");
      out.append(i);
    }
    out.append("}"); ret;
  }
    
  // Need more cases? This should cover all library classes...
  if (name.startsWith("java.") || name.startsWith("javax.")) {
    out.append(str(o)); ret;
  }
    
  String shortName = o.getClass().getName().replaceAll("^main\\$", "");
  
  if (shortName.equals("Lisp")) {
    out.append("l(");
    structureConcept_1(out, getOpt(o, "head"), d);
    L args = cast getOpt(o, "args");
    if (nempty(args))
      for (int i = 0; i < l(args); i++) {
        out.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");
          
        structureConcept_1(out, arg, d);
      }
    out.append(")"); ret;
  }
  
  Class c = o.getClass();
  bool concept = d.conceptClass != null && d.conceptClass.isInstance(o);
  S dynName = shortDynamicClassName(o);
  if (concept && !d.concepts.contains(dynName)) {
    d.concepts.add(dynName);
    if (d.markConcepts)
      out.append("c ");
  }
  
  // serialize an object with fields.
  // first, collect all fields and values in fv.
  
  Map<S, O> fv = new TreeMap;
  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 = dynName;
    fv.remove("className");
  }
  
  S singleField = null;
  
  if (o instanceof ConceptL) {
    out.append(shortName).append("[");
    int l = out.length();
    for (Object x : o/ConceptL.l) {
      if (out.length() != l) out.append(", ");
      structureConcept_1(out, deref(x), d);
    }
    out.append("]");
    ret;
  }
  
  if (o instanceof Str && empty(o/Str.otherNames)) {
    out/*.append("Str ")*/.append(quote(o/Str.name));
    ret;
  }
  
  if (o instanceof Concept) {
    Concept cpt = cast o;
    Set<S> fields = conceptFields(cpt);
    if (l(fields) == 1) singleField = first(fields);
    Concept unl = d.unlisted.get(dynName);
    if (unl == null)
      d.unlisted.put(dynName, unl = unlisted(cpt.getClass()));
    fv.clear();
    for (S field : fields) {
      O val = cget(cpt, field);
      if (val != null && neq(cget(unl, field), val))
        fv.put(field, val);
    }
  }
  
  S fieldOrder = toStringOpt(getOpt(o, "_fieldOrder"));
  
  // Render this$1 first because unstructure needs it for constructor call.
  
  out.append(shortName).append("(");
  int l = out.length();
  
  if (fv.containsKey("this$1")) {
    out.append("this$1=");
    structureConcept_1(out, fv.get("this$1"), d);
    fv.remove("this$1");
  }
  
  // Render the other fields.

  if (fieldOrder != null)
    fv = putKeysFirst(fv, toObjectArray(splitAtSpace(fieldOrder)));
    
  for (S fieldName : keys(fv)) {
    if (out.length() != l) out.append(", ");
    if (neq(fieldName, singleField))
      out.append(fieldName).append("=");
    structureConcept_1(out, fv.get(fieldName), d);
  }

  if (out.length() == l) out.setLength(l-1); // drop "("
  else out.append(")");
}

// drop unused markers
static S structureConcept_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);
}