a big part of the racket project has been about establishing robust tools for transforming syntax. focusing on this use case has resulted in a much more generalized notion of pattern matching and template reconstruction than is typical. these kind of pattern/template combinators, as developed for racket/match and syntax-parse, might appear special-casey at first glance but from use i can attest that they are clean and compositional... they have the appearance of something you'd write informally on a whiteboard, but they really do just work, and the rules are fairly simple and easy to internalize.

Here, the ... pattern combinator means 'match a list (segment) consisting of the pattern Posn(x,y) zero or more times, and because (free variables) x and y are under one ... combinator, bind x to a list of xs and y to a list of ys'. The template combinator ... means 'produce a list (segment) consisting of the template Posn(x, y) zero or more times, returning an error unless x and y are bound to lists. The reason I specified 'under one combinator' for the pattern part is that ellipses can be nested, resulting in contained pattern variables being bound to lists of lists etc. anyway, point is, this is all just compositional rules, with no spooky communication needed between pattern and template. I won't objectively argue that it's not magic, but it's at least not more magic than regexps.