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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 rotate(rule: Rule) -> Rule {
        fn rotate_internal(expr: Expr) -> Expr {
            match expr {
                // TODO: Use box syntax when it gets stabilized.
                Expr::Seq(lhs, rhs) => {
                    let lhs = *lhs;
                    match lhs {
                        Expr::Seq(ll, lr) => {
                            rotate_internal(Expr::Seq(ll, Box::new(Expr::Seq(lr, rhs))))
                        }
                        lhs => Expr::Seq(Box::new(lhs), rhs),
                    }
                }
                Expr::Choice(lhs, rhs) => {
                    let lhs = *lhs;
                    match lhs {
                        Expr::Choice(ll, lr) => {
                            rotate_internal(Expr::Choice(ll, Box::new(Expr::Choice(lr, rhs))))
                        }
                        lhs => Expr::Choice(Box::new(lhs), rhs),
                    }
                }
                expr => expr,
            }
        }

        let Rule { name, ty, expr } = rule;
        Rule {
            name,
            ty,
            expr: expr.map_top_down(rotate_internal),
        }
    }