Parallel Merge Sort

[Josh via Digitalmars-d]

http://pastebin.com/4TyP7Gjj

I feel like there's some sort of optimization that I can do 
instead of the nWayUnion at the end I think that's a lot of 
processing on a single core for the final step.

I was also trying  to use
copy(array[lbound..uperbound].mergeSort, array[lbound..ubound])

instead of:

result ~= array[lbound..ubound].mergeSort

but I'm stuck as to how I can then sort the array now that it 
contained runs
example:
after copy
array would be (thread amount = 3) runs
if threads is = 3 and N = 9
array: [33, 67, 92, 12, 16, 67, 43, 46, 49]

I'm having trouble formulating how I would now sort the runs.
-Or if the copy is a lot of overhead for a large amount of 
numbers.

Thank you,
Josh