pub fn join_maybe_scoped<'a, A, B, RA, RB>( scope: &mut MaybeScope<'a>, oper_a: A, oper_b: B, ) -> (RA, RB)where A: Send + FnOnce(Scope<'a>) -> RA, B: Send + FnOnce(Scope<'a>) -> RB, RA: Send, RB: Send,