)))
                              #'(begin form ...)
                              (next-clause #'(clauses ...))))))))))))
  ;; emacs: (put 'compile-time-case 'scheme-indent-function 1)
  (compile-time-case (system-file-name-convention)
    ((posix)
     (define (file-name-separator? c)
       (char=? c #\/))

     (define file-name-separator-string "/")

     (define (absolute-file-name? file-name)
       (string-prefix? "/" file-name)))

    ((windows)
     (define (file-name-separator? c)
       (or (char=? c #\/)
           (char=? c #\\)))

     (define file-name-separator-string "/")

     (define (absolute-file-name? file-name)
       (define (file-name-separator-at-index? idx)
         (and (> (string-length file-name) idx)
              (file-name-separator? (string-ref file-name idx))))
       (define (unc-file-name?)
         ;; Universal Naming Convention (UNC) file-names start with \\,
         ;; and are always absolute.  See:
         ;;   http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx#fully_qualified_vs._relative_paths
         (and (file-name-separator-at-index? 0)
              (file-name-separator-at-index? 1)))
       (define (has-drive-specifier?)
         (and (>= (string-length file-name) 2)
              (let ((drive (string-ref file-name 0)))
                (or (char<=? #\a drive #\z)
                    (char<=? #\A drive #\Z)))
              (eqv? (string-ref file-name 1) #\:)))
       (or (unc-file-name?)
           (if (has-drive-specifier?)
               (file-name-separator-at-index? 2)
               (file-name-separator-at-index? 0)))))))

(define (in-vicinity vicinity file)
  (let ((tail (let ((len (string-length vicinity)))
                (if (zero? len)
                    #f
                    (string-ref vicinity (- len 1))))))
    (string-append vicinity
                   (if (or (not tail) (file-name-separator? tail))
                       ""
                       file-name-separator-string)
                   file)))