(define (as-string x)
  (cond ((number? x) (number->string x))
        ((symbol? x) (symbol->string x))
        ((string? x) x)
        ((boolean? x) 
            (if x "true" "false"))   ; consider "#t" and "#f" as alternatives
        ((char? x) (char->string x))
        ((list? x)
            (string-append "(" 
               (string-merge (map as-string x) (make-list (- (length x) 1) " "))
               ")"))
        ((vector? x)
          (let ((lst (vector->list x)))
            (string-append "#(" 
               (string-merge (map as-string lst) (make-list (- (length lst) 1) " "))
               ")")))
        ((pair? x)
            (string-append "(" 
               (apply string-append
                  (map (lambda (y) (string-append (as-string y) " ")) (proper-part x))
               )
               " . " (as-string (first-improper-part x))
               ")"))
        (else "??")))