public function getDescendantTokens(callable $shouldDescendIntoChildrenFn = null) {
        foreach ($this->getChildNodesAndTokens() as $child) {
            if ($child instanceof Node) {
                if ($shouldDescendIntoChildrenFn == null || $shouldDescendIntoChildrenFn($child)) {
                    yield from $child->getDescendantTokens($shouldDescendIntoChildrenFn);
                }
            } elseif ($child instanceof Token) {
                yield $child;
            }
        }
    }