@(deffilter incdig ("0" "1") ("1" "2") ("2" "3") ("3" "4") ("4" "5")
                   ("5" "6") ("6" "7") ("7" "8") ("8" "9"))
@(define increment (num out))
@  (local prefix dig junk)
@  (next :string num)
@  (cases)
9
@    (bind out "10")
@  (or)
@*{prefix}@{dig /[0-8]/}
@    (bind out `@prefix@{dig :filter incdig}`)
@  (or)
@*{prefix}9
@    (bind out `@{prefix :filter (:fun increment)}0`)
@  (or)
@junk
@    (throw error `bad input: @junk`)
@  (end)
@(end)
@in
@(increment in out)