version dependencies in MacPorts?

[Jack Howarth]

  After reading through the current MacPorts documentation, I am not
seeing any reference to the ability to set a specific version/revision 
for particular dependencies in a Portfile. I can imagine a number of
instances were this could be a problem. For example, in fink
the gcc44 package has dependencies on cloog and ppl. The minimum
version of cloog required to build gcc45 has increased. How is
this issue addressed in MacPorts? If our gcc44 currently depended
on a cloog 0.15.4 and gcc45 required a newer cloog 0.15.7 release,
how would port able to handle the following case...

1) A user has gcc44 and cloog 0.15.4 installed.
2) A 'sudo port install gcc45' command is executed.

What would trigger having cloog upgraded to the newer 0.15.7
release before the gcc45 build is started? While I could understand
that MacPorts could have issues with version dependencies that
are triggered when installing binary packages, it should at least
be able to make sure that all of packages required for building
another are at the required version/revision, no? Thanks in
advance for any clarifications.
          Jack
ps I can imagine that this might in concept be solved by 
requiring the user to do a selfupdate instead, however you
would still need a mechanism to stage the build order in
a manner that required version/revision dependencies are
satisfied in the correct order.