(define (parse-timestamp input)
  (let ((units '(h m s)))
    (parse
      (bind
        (followed-by
          (sequence
            (map
              (lambda (suffix p)
                (maybe
                  (sequence* ((number (as-string (one-or-more (in char-set:digit))))
                              (_ (char-seq (symbol->string suffix))))
                             (result (* (string->number number) (expt 60 p))))
                  0))
              units
              (reverse (iota (length units)))))
          end-of-input)
        (o result (cut fold + 0 <>)))
      input)))