Re: ellipsis & dot in `syntax-case` pattern

To augment Ryan's answer, the docs for the

 (pattern ... pattern ellipsis pattern ... . pattern)

production specify that the final `pattern` never matches a syntax
object whose datum is a pair.

I'd say there's a slight problem with the documentation here, in that
it assumes that the final `pattern` isn't itself a parenthesized
pattern. I'll fix that.

At Wed, 30 May 2018 20:12:02 -0700, Matthew Butterick wrote:
> According to the syntax-pattern grammar in the docs for `syntax-case` [1] it 
> should be possible to use an ellipsis and a dot in the same syntax pattern. 
> Yet this pattern fails to match the input. Why?
> 
> #lang racket
> (syntax-case #'(a "x" b) ()
>   [(left ... "x" . right) #t]
>   [else #f]) ; #f
> 
> (I know that `syntax-parse` will handle it correctly, but I'm still curious 
> what I'm missing about the behavior of `syntax-case`.)
> 
> #lang racket
> (require syntax/parse)
> (syntax-parse #'(a "x" b)
>   [(left ... "x" . right) #t]
>   [else #f]) ; #t
> 
> 
> [1] 
> https://docs.racket-lang.org/reference/stx-patterns.html?q=syntax-case#%28form.
> _%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29 
> <https://docs.racket-lang.org/reference/stx-patterns.html?q=syntax-case#(form._
> ((lib._racket/private/stxcase-scheme..rkt)._syntax-case))>
> 
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