code: func FilterStringSet(s map[string]bool, f func(x string) bool) map[string]bool {
  	result := map[string]bool{}
  	for k := range s {
  		if f(k) {
  			result[k] = true
  		}
  	}
  	return result
  }
docs: // FilterStringSet accepts a string set `s` (in the form of `map[string]bool`) and a filtering function `f`
  // and returns a string set containing only the strings `x` for which `f(x) == true`