[racket] Rosetta code submission / string operations

I've just submitted a Racket solution to Rosetta Code for "I before E
except after C" (no stretch goal yet).

     http://rosettacode.org/wiki/I_before_E_except_after_C

and noticed a couple of seeming omissions from the basic string operations,
or more likely the documentation.  I ended up rustling up a quick and dirty
test for whether one string includes another as a substring:

(define (in? str sub-str)
  (not (string=? str (string-replace str sub-str ""))))

Is there something I could have used out-of-the-box?  No doubt I could have
used regular expressions, but that seemed like overkill.  I was also unable
to locate code for searching strings.

Other feedback on my rosetta code also most welcome.

-- Dan
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