In Cocoa ordered collection API. IndexSet is used for represented index of the data in ordered collection. So I think it would be great for add the fetch mail API that used IndexSet as the parameter.
Looks like there is a merge conflict here and on line 182.
Sorry about that. Pushed the new code already.
Add Fetching mail with UID or Sequence number Index set.
@jwilling Sorry for the conflicted code. Pushed the fixed.
This is great, thanks!