The sample implementation SRFI-28 appears to require that a dummy
value be supplied whenever the ~% or ~~ escape sequence is used in a format
string, so that, for instance

                            (format "~~~a" #\b)

crashes -- the format string contains two escape sequences but only one
corresponding value.  I suspect that ~% and ~~ should not be counted as
``value-requiring escape sequences'' in the sense intended in the
specification.

        The code for the sample implementation also uses () rather than '()
to denote the empty list, which is non-R5RS.

        I propose instead the following sample implementation of SRFI-28.
It depends on SRFI-6 and SRFI-23.

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(define format
  (lambda (format-string . objects)
    (let ((buffer (open-output-string)))
      (let loop ((format-list (string->list format-string))
                 (objects objects))
        (cond ((null? format-list) (get-output-string buffer))
              ((char=? (car format-list) #\~)
               (if (null? (cdr format-list))
                   (error 'format "Incomplete escape sequence")
                   (case (cadr format-list)
                     ((#\s)
                      (if (null? objects)
                          (error 'format "No value for escape sequence")
                          (begin
                            (write (car objects) buffer)
                            (loop (cddr format-list) (cdr objects)))))
                     ((#\a)
                      (if (null? objects)
                          (error 'format "No value for escape sequence")
                          (begin
                            (display (car objects) buffer)
                            (loop (cddr format-list) (cdr objects)))))
                     ((#\%)
                      (display #\newline buffer)
                      (loop (cddr format-list) objects))
                     ((#\~)
                      (display #\~ buffer)
                      (loop (cddr format-list) objects))
                     (else
                      (error 'format "Unrecognized escape sequence")))))
              (else (display (car format-list) buffer)
                    (loop (cdr format-list) objects)))))))
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