sclass Tree<A> {
  A a;
  L<Tree<A>> children;
  
  *() {}
  *(A *a) {}
  *(A *a, Iterable<Tree<A>> children) { this.children = cloneList(children); }
  
  toString {
    ret empty(children) ? str(a) : a + "\n" + indentx(lines(allToString(children)));
  }
}