// a Lisp-like form
static class Lisp implements Iterable<Lisp> {
  S head;
  L<Lisp> args;
  
  // O more; // additional info, user-defined
  
  *() {}
  *(S head) { this.head = _compactString(head); }
  *(S *head, Lisp... args) {
    argsForEdit().addAll(asList(args));
  }
  *(S head, Collection args) {
    this.head = _compactString(head);
    for (O arg : args) add(arg);
  }
  
  L<Lisp> argsForEdit() {
    ret args == null ? (args = new L) : args;
  }
  
  // INEFFICIENT
  public S toString() {
    if (empty())
      return quoteIfNotIdentifierOrInteger(head);
    new L<S> bla;
    for (Lisp a : args)
      bla.add(a.toString());
    S inner = join(", ", bla);
    if (head.equals(""))
      return "{" + inner + "}"; // list
    else
      return quoteIfNotIdentifier(head) + "(" + inner + ")";
  }

  S raw() {
    if (!isEmpty ()) fail("not raw: " + this);
    ret head;
  }
  
  Lisp add(Lisp l) {
    argsForEdit().add(l);
    ret this;
  }
  
  Lisp add(S s) {
    argsForEdit().add(new Lisp(s));
    ret this;
  }
  
  Lisp add(O o) {
    if (o instanceof Lisp) add((Lisp) o);
    else if (o instanceof S) add((S) o);
    else fail("Bad argument type: " + structure(o));
    ret this;
  }
  
  int size() {
    return l(args);
  }
  
  bool empty() { ret main.empty(args); }
  bool isEmpty() { ret main.empty(args); }
  bool isLeaf() { ret main.empty(args); }
  
  Lisp get(int i) {
    return main.get(args, i);
  }
  
  S getString(int i) { Lisp a = get(i); ret a == null ? null : a.head; }
  
  S s(int i) { ret getString(i); }
  S rawOrNull(int i) {
    Lisp a = get(i); ret a != null && a.isLeaf() ? a.head : null;
  }
  S raw(int i) { ret assertNotNull(rawOrNull(i)); }
  bool isLeaf(int i) { ret rawOrNull(i) != null; }
  
  boolean isA(S head) {
    return eq(head, this.head);
  }

  boolean is(S head, int size) {
    ret isA(head) && size() == size;
  }
  
  bool is(S head) { ret isA(head); }
  bool headIs(S head) { ret isA(head); }
  
  boolean is(S... heads) {
    return asList(heads).contains(head);
  }
  
  // check head for one of these (ignore case)
  boolean isic(S... heads) {
    return containsIgnoreCase(heads, head);
  }
  
  public Iterator<Lisp> iterator() {
    return main.iterator(args);
  }
  
  Lisp subList(int fromIndex, int toIndex) {
    Lisp l = new Lisp(head);
    l.argsForEdit().addAll(args.subList(fromIndex, toIndex)); // better to copy here I guess - safe
    return l;
  }

  public bool equals(O o) {
    if (o == null || o.getClass() != Lisp.class) ret false;
    Lisp l = cast o;
    ret eq(head, l.head) && (isLeaf() ? l.isLeaf()
      : l.args != null && eq(args, l.args));
  }
  
  public int hashCode() {
    ret head.hashCode() + main.hashCode(args);
  }
  
  Lisp addAll(L args) {
    for (O arg : args) add(arg);
    ret this;
  }
  
  S unquoted() { ret unquote(raw()); }
  S unq() { ret unquoted(); }
  S unq(int i) { ret get(i).unq(); }
  
  // heads of arguments
  L<S> heads() { ret collect(args, "head"); }
}