;;;
;;; Destructively  replaces character  '#' with  ';'. At  the  time of
;;; writing semicolons were used only in #-commented sections.
;;;
;;; FIXME: unfortunately there is at  least one use of the #-character
;;; in  atomic class  symbol ---  C#.   See atom  types 897  & 898  in
;;; oplsaa.prm and smoothaa.prm. It happens  that this C# atom type is
;;; the only  place where the  char # is  not preceeded by a  space or
;;; another # (cf.  grep "[^ #]#" *.prm).
;;;
(define (rewrite! str)
  (string-map! (lambda (c)
                 (case c
                   ((#\#) #\;)          ; # -> ;
                   (else c)))           ; leave the rest alone
               str))

(define (rewrite!! str)
  (let ((n (string-length str)))
    (let loop ((i 0)                    ; pointer into the string
               (f #t)) ; should we interprete evential # as a comment sign?
      (if (< i n)
          (let ((c (string-ref str i))
                (i++ (+ 1 i)))
            (if (and f (equal? c #\#))
                (begin
                  (string-set! str i #\;)
                  (loop i++ #t))
                (loop i++ (char-whitespace? c))))))))

(define a (string-copy "c# #xyz# ## c# # ## ### comm#nts"))
(rewrite!! a)
(pretty-print a)
(exit 1)