{Record is_letter is_digit is_upper is_lower}

Where

Let (is_letter c)
    Or (is_lower c) (is_upper c)

Where

Let (is_digit c)
    And ('0' <= c) (c <= '9')

Let (is_upper c)
    And ('A' <= c) (c <= 'Z')

Let (is_lower c)
    And ('a' <= c) (c <= 'z')