Function peresil::combinators::optional_append

pub fn optional_append<P, E, S, A, F, T>(
    append_to: A, 
    parser: F
) -> impl FnOnce(&mut ParseMaster<P, E, S>, P) -> Progress<P, Vec<T>, E> where
    F: FnOnce(&mut ParseMaster<P, E, S>, P) -> Progress<P, T, E>,
    A: IntoAppend<T>,
    P: Point,