build place cleanup

[Christoph Biedl <debian.packages.hhqj@manchmal.in-ulm.de>]

Hello,

a while ago I learned at a place I cannot remember that the clean
target should restore the build place, more precisely:  The state
created after an initial unpacking ("dpkg-source -x" or the like) and
"fakeroot debian/rules clean" should be restored after a build (debuild
or the like) and clean (again: "fakeroot debian/rules clean") cycle.

While this makes a lot of sense I am not aware how strong this
"should" is or where this rule is written down.  Is this fully covered
by "4.9 Main building script", thus as "must"?  I think I saw several
packages that would violate that.

Thanks,

    Christoph