Is xz implicit BuildRequires?

[Petr Pisar]

Hello,

I have a package where upstream provides tar balls compressed by xz.
Building in koji (F15) works as xz is part of group `build' or it's
pulled by rpmbuild (can be seen in root.log). However Packaging
Guidelines: BuildRequires: Exception
<https://fedoraproject.org/wiki/Packaging/Guidelines#Exceptions_2> does
not list the xz package.

Is xz implicit and is not just listed in guidelines by a mistake? Or do
I have to BuildRequire xz package explicitelly?

-- Petr