type Strings map[string]stdext.Unit
  […]
  func (a Strings) Intersection(b Strings) Strings {
  	result := NewStrings()
  	for as, _ := range a {
  		for bs, _ := range b {
  			if as == bs {
  				result.Add(as)
  				break
  			}
  		}
  	}
  	return result
  }