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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 concatenate(rule: Rule) -> Rule {
        let Rule { name, ty, expr } = rule;
        Rule {
            name,
            ty,
            expr: expr.map_bottom_up(|expr| {
                if ty == RuleType::Atomic {
                    // TODO: Use box syntax when it gets stabilized.
                    match expr {
                        Expr::Seq(lhs, rhs) => match (*lhs, *rhs) {
                            (Expr::Str(lhs), Expr::Str(rhs)) => Expr::Str(lhs + &rhs),
                            (Expr::Insens(lhs), Expr::Insens(rhs)) => Expr::Insens(lhs + &rhs),
                            (lhs, rhs) => Expr::Seq(Box::new(lhs), Box::new(rhs)),
                        },
                        expr => expr,
                    }
                } else {
                    expr
                }
            }),
        }
    }