{
: is_letter
: is_digit
: is_upper
: is_lower
}

Where

Define (is_letter c)
    Or (is_lower c) (is_upper c)

Where

Define (is_digit c)
    And [`0` <= c] [c <= `9`]

Define (is_upper c)
    And [`A` <= c] [c <= `Z`]

Define (is_lower c)
    And [`a` <= c] [c <= `z`]