Nobody Else Has It

A sticker-swap club wants to know which stickers are rare. groups is a list with one entry per collector, and each entry is a list of the stickers they own (with repeats).

Task: write unique_to_each(groups). Return a list with one entry per collector, in the same order. Each entry is a list, sorted alphabetically, of the stickers that only that collector owns — nobody else in groups has one.

• A collector with nothing rare gets [].
• If groups is empty, return [].