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    // pest. The Elegant Parser
    // Copyright (c) 2018 Dragoș Tiselice
    //
    // Licensed under the Apache License, Version 2.0
    // <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0> or the MIT
    // license <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. All files in the project carrying such notice may not be copied,
    // modified, or distributed except according to those terms.

    use crate::ast::*;

    pub fn list(rule: Rule) -> Rule {
        let Rule { name, ty, expr } = rule;
        Rule {
            name,
            ty,
            expr: expr.map_bottom_up(|expr| {
                // TODO: Use box syntax when it gets stabilized.
                match expr {
                    Expr::Seq(l, r) => match *l {
                        Expr::Rep(l) => {
                            let l = *l;
                            match l {
                                Expr::Seq(l1, l2) => {
                                    // Converts `(rule ~ rest)* ~ rule` to `rule ~ (rest ~ rule)*`,
                                    // avoiding matching the last `rule` twice.
                                    if l1 == r {
                                        Expr::Seq(l1, Box::new(Expr::Rep(Box::new(Expr::Seq(l2, r)))))
                                    } else {
                                        Expr::Seq(Box::new(Expr::Rep(Box::new(Expr::Seq(l1, l2)))), r)
                                    }
                                }
                                expr => Expr::Seq(Box::new(Expr::Rep(Box::new(expr))), r),
                            }
                        }
                        expr => Expr::Seq(Box::new(expr), r),
                    },
                    expr => expr,
                }
            }),
        }
    }