[racket] [htdp] Can this be simplified?

Here is my solution to a modified version of DNAprefix from exercise 
17.6.6. I feel like it *should* be able to be written more 
clearly/simply but I can't think of the logic to make it work properly:

Modify |DNAprefix| so that it returns the first item beyond the pattern 
in the search-string if the pattern is a proper prefix of the 
search-string. If the lists do not match or if the pattern is no shorter 
than the search-string, the modified function should still return 
|false|. Similarly, if the lists are equally long and match, the result 
is still |true|.

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