func usage(_ e:String) {
  print("error: \(e)")
  print("./caeser -e 19 a-secret-string")
  print("./caeser -d 19 tskxvjxlskljafz")
}

func charIsValid(_ c:Character) -> Bool {
  return c.isASCII && ( c.isLowercase || 45 == c.asciiValue )