Using materialized values to derive ordering for MergeSorted


I have multiple sorted Sources[A, M] which I want to merge using MergeSorted. However, MergeSorted needs an Ordering[A], which will depend on the materialized values of the input sources. I know how to get Outlets for the materialized values of the sources, but I don’t know how use those to derive an Ordering for MergeSorted. I cannot prematerialize, because I don’t yet have a Materializer.

How can I proceed?