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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 skip(rule: Rule) -> Rule {
        fn populate_choices(expr: Expr, mut choices: Vec<String>) -> Option<Expr> {
            match expr {
                Expr::Choice(lhs, rhs) => {
                    if let Expr::Str(string) = *lhs {
                        choices.push(string);
                        populate_choices(*rhs, choices)
                    } else {
                        None
                    }
                }
                Expr::Str(string) => {
                    choices.push(string);
                    Some(Expr::Skip(choices))
                }
                _ => None,
            }
        }

        let Rule { name, ty, expr } = rule;
        Rule {
            name,
            ty,
            expr: if ty == RuleType::Atomic {
                expr.map_top_down(|expr| {
                    // TODO: Use box syntax when it gets stabilized.
                    if let Expr::Rep(expr) = expr.clone() {
                        if let Expr::Seq(lhs, rhs) = *expr {
                            if let (Expr::NegPred(expr), Expr::Ident(ident)) = (*lhs, *rhs) {
                                if ident == "ANY" {
                                    if let Some(expr) = populate_choices(*expr, vec![]) {
                                        return expr;
                                    }
                                }
                            }
                        }
                    };

                    expr
                })
            } else {
                expr
            },
        }
    }