I've recently been implementing a syntax impander -- in addition to expanded form, the syntax expander returned the information about the transformations that it used, thanks to which the syntax expansion process could be reverted.

Since I was using my transformer to perform "Y-lining" of definitions, my code of the form
(define f1 <v1>)
(define f2 <v2>)
...
(define fn <vn>)
expression

was supposed to get transformed into something that begins with
(let ((f1 (lambda ((f1 f2 ... fn)) f1))
      (f2 (lambda ((f1 f2 ... fn)) f2))
      ...
      (fn (lambda ((f1 f2 ... fn)) fn)))
  <inner-context>)

that is, f1 ... fn were re-bound to name tuple selectors in the <inner-context> (the code works because I modified Scheme to perform destructured bindings in arguments to lambda).

I've noticed that while syntax-rules allows such forms as templates, it does not allow them as patterns (the result of expansion would fail to match).
However, it is possible to modify the pattern matcher to handle cases like this as well, which I did. (By the way, the fact that syntax-rules does not allow the same identifier to appear more than once in a pattern also seems like an unnatural and arbitrary limitation)

However, if we allow the consecutive ellipses, as in the proposal, the templates and patterns are no longer symmetrical. And while of course the semantics of the consecutive ellipses in templates are indeed very intuitive, I think we should ask the question whether they should also be allowed in templates (I guess they would need to match lists of lists). However, if we decided to do so, i.e. allow , I think that it would make the "impansions" of macros more difficult: once the form '((1 2 3) (4) (5 6)) is concatenated to (1 2 3 4 5 6), there is no way of transforming it back without storing additional information regarding the shape of the original list.

Another question that comes to my mind is whether one could give any examples of some practical application of these "consecutive ellipses" templates", or is it just a purely theoretical construct at this moment?