adding proof of context-sensitivy of raw string literals

Stumbled across this and thought it would be cool to prove. I've never used Ogden's Lemma before, but I'm pretty sure I used it right. The pumping lemma definitely doesn't seem sufficient for the job. In particular, when using the pumping lemma, you can always just pump one of the quotes, and it's fine. Ogden's Lemma lets you effectively force the pumper to use certain characters in the string.

@cmr
FYI i prefer wikipedia's phrasing of the lemma, in particular its use of the word "positions" to refer to what is being marked; the phrase "`p` characters" is more ambiguous in its interpretation (i.e. w.r.t. whether to count multiple occurrences of the same character).