Function join 

pub fn join<A, B, RA, RB>(oper_a: A, oper_b: B) -> (RA, RB)
where
    A: FnOnce() -> RA,
    B: FnOnce() -> RB,

Runs oper_a then oper_b in sequence, unlike rayon’s join, which may run them concurrently on separate threads.