I would be more radical and add them as a part of the language, the way map is now. AFAIK, Wirthian languages like Pascal and Oberon all have sets as built-ins,

I would be more radical and add them as a part of the language, the way map is now. AFAIK, Wirthian languages like Pascal and Oberon all have sets as built-ins, with +, -, and * operators defined on them, and Go is heavily inspired by them.