port with different kinds of distfiles

[Michael Allman]

Hello,

I'm attempting to augment the lang/scala2.10 Portfile with a +doc variant 
for installing the api docs. The scala api docs come in two dist 
formats---zip and txz. The main scala distfile itself is a tgz. You can 
find the download links here:

http://www.scala-lang.org/downloads

I'm trying to add the scala doc txz distfile to the +doc variant, but I 
can't figure out how to configure the port to use xz to extract the doc 
distfile and tar for the main distfile. Can anyone suggest a clean 
solution?

Cheers,

Michael