You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: lib.rs
    path of file: ./repos/rust99/multiway-trees/P73/src
    
    the code of the file until where you have to start completion: use mtree::MTree;

pub fn lispy_tree(tree: &MTree) -> String {
    if tree.get_children().is_empty() {
        String::from(tree.get_value())
    } else {
        let mut s = String::new();
        s.push('(');
        s.push(tree.get_value());
        for child in tree.get_children() {
            s.push(' ');
            s.push_str(&lispy_tree(child));
        }
        s.push(')');
        s
    }
}

/// Parse lispy string and construct MTree
/// BNF syntax:
/// <Value>    ::= Character
/// <Node>     ::= <Value> | '(' <Children> ')'
/// <Children> ::= <Node> | <Node> ' ' <Children>
pub fn lispy_str_to_tree(s: &str) -> MTree {
    fn parse_node(s: &str) -> (MTree, &str) {
        let (c, last) = next_token(s);
        match c {
            '(' => {
                let (v, last) = next_token(last); /
    
pub fn lispy_str_to_tree(s: &str) -> MTree {
    fn parse_node(s: &str) -> (MTree, &str) {
        let (c, last) = next_token(s);
        match c {
            '(' => {
                let (v, last) = next_token(last); // value of this node
                let (c, last) = next_token(last);
                assert!(c == ' ', "Invalid lispy string"); // next token shoud be a white space
                let (children, last) = parse_children(last);
                let (c, last) = next_token(last);
                assert!(c == ')', "Invalid lispy string"); // next token shoud be a right parentheses
                (MTree::node(v, children), last)
            }
            ' ' => parse_node(last),
            _ => (MTree::leaf(c), last),
        }
    }

    fn parse_children(s: &str) -> (Vec<MTree>, &str) {
        let mut res = vec![];
        let (node, last) = parse_node(s);
        res.push(node);

        let (c, _) = next_token(last);
        match c {
            ')' => (res, last),
            _ => {
                let (nodes, last) = parse_children(last);
                for node in nodes {
                    res.push(node);
                }
                (res, last)
            }
        }
    }

    fn next_token(s: &str) -> (char, &str) {
        let (first, last) = s.split_at(1);
        let c = first.chars().nth(0).unwrap();
        (c, last)
    }

    let (tree, rem) = parse_node(s);
    assert!(rem == "", "Invalid lispy string");
    tree
}