pub fn insert(trie: Trie, string: &str) -> Trie {
    match trie {
        Trie::Leaf => Trie::Node(vec![(string.to_owned(), Trie::Leaf)]),
        Trie::Node(mut children) => {
            if let Some((_label, _edge)) = search_prefix(&children, string) {
                return unimplemented!();
            }
            children.push((string.to_owned(), Trie::Leaf));
            Trie::Node(children)
        }
    }
}