}

        @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 {