[R6RS] "Unpredictable behavior" with `quasiquote'

[Michael Sperber]

As Matthew pointed out to me, we had this holdover from R5RS:

 Unpredictable behavior can result if any of the identifiers
 quasiquote, unquote, or unquote-splicing appear in positions within a
 qq template otherwise than as described above.

I've made that into a syntax defect.  If you object, post.

-- 
Cheers =8-} Mike
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