I described it in this post http://rystsov.info/2015/09/16/how-paxos-works.html

With single-decree Paxos you can write a storage which provides the following API:

function changeQuery(key, change, query) {

var value=this.db.get(key); value = change(value);

this.db.commit(key, value);

return query(value);

}

By providing different implementations of change & query you can achieve different beviour including CAS.