Data.List.NonEmpty
https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-List-NonEmpty.html
How do I flatten a NonEmpty of NonEmptys -- staying within NonEmpty? IOW I do not want to convert to regular list and do a concat and then convert back to NonEmpty.
As NonEmpty is a Monad you can use join:
join :: Monad m => m (m a) -> m a
Specialized to NonEmpty:
join :: NonEmpty (NonEmpty a) -> NonEmpty a
You could use sconcat from Data.Semigroup. It combines the elements of a nonempty list using <>, which in this case means concatenation.
(It's found there rather than in Data.List.NonEmpty because it's a method of the Semigroup class.)