code: func (s stringSet) isSubsetOf(t stringSet) bool {
  	j := 0
  	for _, ss := range s {
  		for j < len(t) && t[j] < ss {
  			j++
  		}
  		if j >= len(t) || t[j] != ss {
  			return false
  		}
  	}
  	return true
  }
docs: // isSubsetOf returns true if all strings in s are also in t.
  // It assumes both sets are sorted.