How about old_value_set = set(s.lower() for s in old_value) instead of map?

That could even use set comprehension:

{s.lower() for s in old_value}

If you'd like, I'll begin working on a PR to convert the uses of map() to use list, dict, or set comprehension instead. I'll do this as separate work though.

How about old_value_set = set(s.lower() for s in old_value) instead of map?

Done in #142