}

        @VisibleForTesting
        List<String> flatten() {
            final List<String> prefixes = new ArrayList<String>();
            rootNode.visitHierarchy(0, new NodeVisitor() {
                @Override
                public void visitNode(int depth, Node node) {
                    if (depth == 0) {
                        prefixes.add(node.prefix);
                    } else {

     *
     * @param visitor the visitor to call back, must not be {@code null}
     * @return {@code true} to visit siblings nodes of this node as well, {@code false} to skip siblings
     */
    boolean accept(@Nonnull NodeVisitor visitor);

    /**
     * Returns a new tree starting at this node, filtering the children.
     * Note that this node will not be filtered and only the children
     * and its descendant will be checked.
     *