Function peresil::combinators::optional

pub fn optional<P, E, S, F, T>(
    parser: F
) -> impl FnOnce(&mut ParseMaster<P, E, S>, P) -> Progress<P, Option<T>, E> where
    F: FnOnce(&mut ParseMaster<P, E, S>, P) -> Progress<P, T, E>,
    P: Point,
    E: Recoverable,