static S nlUnparse(Lisp tree) {
  ret nlUnparse(tree, true);
}

static S nlUnparse(Lisp tree, boolean dropOuterBrackets) {
  new StringBuilder buf;
  nlUnparse(tree, buf, dropOuterBrackets);
  ret str(buf);
}

static void nlUnparse(Lisp tree, StringBuilder buf, boolean dropOuterBrackets) {
  if (tree == null) ret;
  
  // special classes - Var etc.
  if (tree.getClass() != Lisp.class) {
    buf.append(tree);
    ret;
  }
  
  if (tree.isA("[]")) {
    if (!dropOuterBrackets) buf.append("[");
  } else if (tree.isA("()"))
    buf.append("(");
  else {
    if (tree.isLeaf())
      buf.append(tree.head);
    else
      buf.append("?? " + tree);
    ret;
  }
  
  for (int i = 0; i < tree.size(); i++) {
    if (i != 0) buf.append(" ");
    nlUnparse(tree.get(i), buf, false);
  }
  
  if(tree.isA("[]")) {
    if (!dropOuterBrackets) buf.append("]");
  } else
    buf.append(")");
}