Yield combinations from start to self in short lexicographic order.

def powerset(self, start=None, excludestart=False):
        """Yield combinations from start to self in short lexicographic order."""
        if start is None:
            start = self.infimum
            other = self.atoms()
        else:
            if self | start != self:
                raise ValueError('%r is no subset of %r' % (start, self))
            other = self.fromint(self & ~start).atoms()
        return map(self.frombitset, combos.shortlex(start, list(other)))