;; A list of characters considered as blank space characters
(define white-space-char-list
   (list #\space (as-char 13) (as-char 10) #\tab))

;; Is the string str empty or blank (consists of white space)
(define (blank-string? str)
  (or (empty-string? str) 
      (string-of-char-list? str white-space-char-list)))

;; Returns if the string str is numeric.
;; More specifically, does str consist exclusively of the
;; ciffers 0 through 9. 
(define (numeric-string? str)
  (string-of-char-list? str 
    (list #\0 #\1 #\2 #\3 #\4 #\5 #\6 #\7 #\8 #\9 )))