;--- String manipulation

(define (string-suffix? src suffix)
  (let [(src-len (string-length src)) (suf-len (string-length suffix))]
    (and 
      (>= src-len  suf-len) 
      (string=? suffix (substring src (- src-len suf-len) src-len))
)))

(define (string-prefix? src prefix)
  (let [(src-len (string-length src)) (pref-len (string-length prefix))]
    (and
      (>= src-len pref-len)
      (string=? prefix (substring src 0 pref-len))
)))

(define (string-contains? src elem) ; we use port logic to implement this one
  (seek-until (open-input-string src) elem))