{
: 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`])