code: func (k Key) IsUseable() bool {
  	if !k.ExpirationDate.IsZero() && k.ExpirationDate.Before(time.Now()) {
  		return false
  	}
  	switch k.Validity {
  	case "m":
  		return true
  	case "f":
  		return true
  	case "u":
  		return true
  	}
  	return false
  }
docs: // IsUseable returns true if GPG would assume this key is useable for encryption