<dblack / superlink.net> wrote
> Namely, the idea of an extensible %_{} syntax, so that one could
> define the behavior of %y{}, %x{}, and so on from within a Ruby
> program or extension.

In a slightly more general context, a worked-out approach to extensible
syntax grammars is described in:

http://www.hpl.hp.com/techreports/Compaq-DEC/SRC-RR-121.pdf