@(do (defun inc-num-str (str-in)
       (let ((len (length str-in))
             (str (copy-str str-in)))
         (for ((i (- len 1)))
              ((>= i 0) `1@str`)
              ((dec i))
           (if (<= (inc [str i]) #\9)
             (return str)
             (set [str i] #\0))))))
@(bind a @(inc-num-str "9999"))
@(bind b @(inc-num-str "1234"))