oneLetterBetween :: String -> Bool
oneLetterBetween (x:y:z:xs)
  | x == z    = True
  | otherwise = oneLetterBetween (y:z:xs)
oneLetterBetween :: String -> Bool
oneLetterBetween (x:y:z:xs)
  | x == z    = True
  | otherwise = oneLetterBetween (y:z:xs)
oneLetterBetween (x:y:z:xs)
  | x == z    = True
  | otherwise = oneLetterBetween (y:z:xs)