Function parallel_reduce 

pub fn parallel_reduce<T, R, Fold, Combine>(
    data: &[T],
    identity: R,
    fold: Fold,
    combine: Combine,
    n_threads: usize,
) -> CoreResult<R>
where
    T: Sync + 'static,
    R: Send + Clone + 'static,
    Fold: Fn(R, &T) -> R + Send + Sync + 'static,
    Combine: Fn(R, R) -> R,

Parallel reduction using a chunk-local reduce followed by a sequential combine across chunks.

• fold: combine one element T into accumulator R (called per thread).
• combine: merge two accumulators (called sequentially to merge chunks).
• identity: neutral element for combine.