static <A> LL<A> transpose(LL<A> l) {
  new LL<A> cols;
  for i over l: {
    L<A> row = l.get(i);
    for j over row: {
      while (j >= l(cols)) cols.add(new L);
      listPut(cols.get(j), i, row.get(j));
    }
  }
  ret cols;
}