The Rare Ones

A pet shop survey asks customers which pet they own. A few answers, like axolotl, turn up only once or twice. Before the answers go into a model, the shop wants to lump the rare ones together, so first it needs to know which ones are rare.

Task: write rare_categories(answers, min_share).

• answers is a list of strings. A missing answer is None.
• Missing answers are left out completely. Shares are worked out from the answers that are there.
• An answer is rare when its share of those answers is less than min_share (a number between 0 and 1). A share exactly equal to min_share is not rare.
• Return a list of the rare answers, each one once, sorted alphabetically. If nothing is rare, return an empty list.