template<class Node>
void printTree(const Node& tree, const string& indent = "", bool last = true)
{
    cout << indent << "+- " << tree.to_s() << endl;
    auto new_indent = indent + ( last ? "   " : "|  ");

    for (int i = 0; i < tree.child_size(); ++i) {
        bool last = i == (tree.child_size() - 1);
        printTree( tree.childs()[i], new_indent, last);
    }
}
                                          
// call with
printTree( tree);

+- root
   +- branch-A
   |  +- sibling-X
   |  |  +- grandchild-A
   |  |  +- grandchild-B
   |  +- sibling-Y
   |  |  +- grandchild-C
   |  |  +- grandchild-D
   |  +- sibling-Z
   |     +- grandchild-E
   |     +- grandchild-F
   +- branch-B
      +- sibling-J
      +- sibling-K