Function maybe_par_map_ct_progress 

pub fn maybe_par_map_ct_progress<T, F>(
    n: usize,
    threshold: usize,
    token: Option<&CancelToken>,
    progress: Option<&ProgressReporter>,
    f: F,
) -> Option<Vec<T>>
where
    F: Fn(usize) -> T,

Sequential fallback: identical output to the parallel version.