code: func (kl KeyList) UseableKeys() KeyList {
  	nkl := make(KeyList, 0, len(kl))
  	for _, k := range kl {
  		if !k.IsUseable() {
  			continue
  		}
  		nkl = append(nkl, k)
  	}
  	sort.Sort(nkl)
  	return nkl
  }
docs: // UseableKeys returns the list of useable (valid keys)