How to implement mergesort with parallel_reduce?

Hello,
I am trying to implement mergesort with parallel_reduce. Let us say that the range [ P, R ) is split into two:
[ P, Q ) and [Q, R )

I would like to recursively divide these mergesort these two intervals and then merge the two.

The problem is, the subtask does not seem to let the user access the ranges of the two tasks.

Now my question is, is it possible to implemet these really recursive functions like mergesort and quicksort with parallel_reduce? If so, could someone give me a hint to approach the problem please?

Thanks,
Sviiya