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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.

    // This structure serves to improve performance over Token objects in two ways:
    //
    //   * it is smaller than a Token, leading to both less memory use when stored in the queue but also
    //     increased speed when pushing to the queue
    //   * it finds its pair in O(1) time instead of O(N), since pair positions are known at parse time
    //     and can easily be stored instead of recomputed
    #[derive(Debug)]
    pub enum QueueableToken<R> {
        Start {
            end_token_index: usize,
            input_pos: usize,
        },
        End {
            start_token_index: usize,
            rule: R,
            input_pos: usize,
        },
    }