Just throwing this out there - the Extend instance is a little bonkers, so we can just close this if it seems nonsensical.

I feel like the Extend instance really should operate on each subset of the thing, so if it was a 1 :| [2, 3, 4] it would call f on 1 :| [2, 3, 4], 2 :| [3, 4], 3 :| [4], 4 :| [], but unless we invented some kind of Unconsable class I don't think there's a way of expressing that. Given that, extend operates on every element with an empty tail instead ¯_(ツ)_/¯.

edit: I went with Plus as the constraint since elsewhere we prefer Alternative over Monoid classes.