bug#73290: sxml-match doesn't support several ellipsis at the same level

This is as much a feature request as a bug.

sxml-match doesn't seem to support several ellipsis at the same level,
which is common in xml.

See the following example for a minimal reproducible example (using
guile 3.0.9-0.3b76a30).

```
(use-modules (sxml match))

(define xml '(d (a 1) (a 2) (a 3) (b 4) (b 5) (b 6)))

(sxml-match
 xml
 ((d (a ,i) ... (b ,j) (b ,k) (b ,l))
  (list (list i ...) (list j k l))))

(sxml-match
 xml
 ((d (a ,i) ... (b ,j) ...)
  (list (list i ...) (list j ...))))  
```

Expected : (list (list 1 2 3) (list 4 5 6)) for both cases

In reality : 
(list (list 1 2 3) (list 4 5 6)) in the first case
Error "ellipses not allowed in this context" in the second case

-- 
Best regards,
Nicolas Graves