Bug#630787: can not update/install morituri because of a syntaxerror

[Fabian Greffrath]

Am 08.08.2011 10:44, schrieb Jonas Smedegaard:
> We do not solve this issue by educating Debian packages more widely
> about weird possible combinations of packages: Debian support upgrades
> one stable release at a time - Debian do *not* support keeping around
> old packages!

It is exact this point that I abut on. Is this principle documented 
somewhere, e.g. in Debian policy?