{ : 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`])