Yes, is because of the formatting (+ operator). One option is to make Cassanity.add an alias of Cassanity.inc.
With a custom add method, doing `Cassanity.add('a@b.com')` or `Cassanity.add(['a@b.com', 'b@c.com'])` looks good, but what do we do with lists? Lists have a different formatting.

What about sadd/sremove for sets, and add/remove for lists?