Why {{}} for regexp closures instead of just {}?

[Chris Dolan]

S05 always uses single curlies for closures, but throughout Parrot,  
code seems to use double curlies in PGE regexps.  Why is that?

That is, why this:
  m/ foo {{ say "found foo" }} /
and not this:
  m/ foo { say "found foo" } /
The latter complains about "Statement not terminated properly".

Chris