isHexChar :: Char -> Bool
isHexChar ch
    | ch >= '0' && ch <= '9' = True
    | ch >= 'A' && ch <= 'F' = True
    | ch >= 'a' && ch <= 'f' = True
    | otherwise              = False
isHexChar :: Char -> Bool
isHexChar ch
    | ch >= '0' && ch <= '9' = True
    | ch >= 'A' && ch <= 'F' = True
    | ch >= 'a' && ch <= 'f' = True
    | otherwise              = False
isHexChar ch
    | ch >= '0' && ch <= '9' = True
    | ch >= 'A' && ch <= 'F' = True
    | ch >= 'a' && ch <= 'f' = True
    | otherwise              = False