ordered words

An ordered word has its letters in alphabetical order. Write a function that returns the longest ordered word in a dictionary file.

 $ cat low.scm 
 (use-modules (ice-9 rdelim)) 
 (define (ordered-word wd) 
   ; Return true iff the letters in the given word are in  
   ; alphabetical order. 
   (let ((n (- (string-length wd) 1))) 
     (if (< n 1) 
       (let loop ((fst-i (- n 1)) (snd-c (string-ref wd n))) 
         (if (< fst-i 0) 
           (let ((fst-c (string-ref wd fst-i))) 
             (if (char>? fst-c snd-c) 
               (loop (- fst-i 1) fst-c)))))))) 
 (define (longest-ordered-word dict-file) 
   ; Return the longest ordered word in the given file. 
   (call-with-input-file dict-file 
     (lambda (p) 
       (let loop ((wd (read-line p)) (low "") (low-n 0)) 
         (if (eof-object? wd) 
           (let ((n (string-length wd)) 
                 (next-wd (read-line p))) 
             (if (and (> n low-n) (ordered-word wd)) 
               (loop next-wd wd n) 
               (loop next-wd low low-n)))))))) 
 (format #t "~a~%" (longest-ordered-word "/usr/share/dict/words")) 
 $ guile low.scm