public BucketSet union(BucketSet rhs) {
        if (rhs == null) {
            return null;
        } else {
            BucketSet ret = new BucketSet(this);
            ret.addAll(rhs);
            return ret;
        }
    }
Returns the union between this bucket set and another.

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