The merge-sort algorithm for sorting a list is based on dividing the list into smaller lists, then

sorting each smaller list and finally merging these sorted smaller lists into one resulting list.

Accordingly, to sort list L:

- Divide L into two lists L1 and L2 of approximately equal length

- Sort L1 and L2 (using merge-sort) giving S1 and S2 respectively

- Merge S1 and S2 giving the sorted list R.

Write a PROLOG implementation of merge-sort.