virtual package question

[Miles Fidelman <mfidelman@meetinghouse.net>]

Folks,

As I dig more into the whole question of installing alternate init 
systems, I realize that I'm a bit confused about the behavior of apt- 
when it comes to virtual packages; and I can't find any documentation 
that clarifies things.

Specifically, in this situation:

Package: A
Depends: B

Package: B1
Provides: B
Conflicts: B
Replaces: B

Package B2
Provides: B
Conflicts: B
Replaces: B

As I understand it, the package priority determines what gets installed 
by default, but how does one explicitly install a lower-priority 
alternative, and what, if anything, prevents the higher priority 
alternative from being reinstalled during a future install (e.g, as part 
of a dependency fulfillment or an updgrade)?

I know I ran across this at some point when installing Postfix - but I 
can't for the life of me remember how this all worked out, an can't find 
documentation anywhere.

Can someone elaborate for me?

Thanks!

Miles


--
In theory, there is no difference between theory and practice.
In practice, there is.   .... Yogi Berra