static Lisp listToSNLTree(L list) {
  Lisp tree = lisp(snlCommaOp(l(list)));
  for (O s : list)
    if (s instanceof Lisp)
      tree.add((Lisp) s);
    else if (s instanceof S)
      tree.add(quote((S) s));
    else
      tree.add("?");
  ret tree;
}