automake (and possibly others) now use .tar.xz, but nothing ensures that something to extract .xz is available (Bug #408)
Either we need to add some kind of lzma-utils to basesystem, or the spells that are .xz need to explicitly depend on some lzma extraction utility
This is in current stable!
Doesn't sorcery ensure basesystem and its dependencies is cast first? Or is that only true when $FORCE_BASESYSTEM_DEPENDS=on?
As an alternative, I suppose sorcery can recast basesystem (or maybe cleanse --fix basesystem) if it can't find the proper extraction utility or at least notify the user to recast basesystem.