Ouch!

  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
  }

Reminded me of https://accidentallyquadratic.tumblr.com.

Ick, good point. Since the fake set is a map, that should use key lookup?

Yes. Basically:

  for k := range s1 {
    if s2.Contains(k) {
      result.Add(k)
    }
  }