code: func (rs RuleSet) Diff(other RuleSet) RuleSet {
  	result := NewRuleSet()
  	for i := range rs {
  		if !other.Contains(rs[i]) {
  			result.Add(rs[i])
  		}
  	}
  	return result
  }
docs: // Diff returns a new RuleSet containing rules in rs that are not in other.