sclass Tree<A> {
  A a;
  L<Tree<A>> children;
  
  *() {}
  *(A *a) {}
  *(A *a, Iterable<Tree<A>> children) { this.children = cloneList(children); }
  
  toString { new LS out; toString(out, 0); ret lines(out); }
  
  // Note: This doesn't call the children() method
  void toString(LS out, int indent) {
    out.add(spaces(indent) + str(a));
    for (Tree<A> t : unnull(children))
      t.toString(out, indent+2);
  }
  
  A value() { ret a; }
  
  // if you override, make sure to set field children
  L<Tree<A>> children() { ret children; }
}