public BucketSet intersection(BucketSet rhs) {
        if (rhs == null) {
            return new BucketSet(this); // The other has all buckets marked, this is the smaller.
        } else {
            BucketSet ret = new BucketSet(this);
            ret.retainAll(rhs);
            return ret;
        }
    }
Returns the intersection between this bucket set and another.

@param rhs The set to form an intersection with.
@return The intersection.
document/src/main/java/com/yahoo/document/select/BucketSet.java