reinstate package list

[greg wm]

whenever i install or update any packages, i save a new complete listing 
of all the packages i have currently installed.

now i want to revert to an earlier state, using the package list of the 
state i want to revert to.  i've already set up channels pointing into 
/var/cache/yum/*/packages, and have succeeded already to perform a 
simple downgrade of a single package using smart.

can smart take my entire package list and restore the given state in one 
go?  how exactly?

the package list file contains 1425 packages, 31458 bytes, head:
4Suite-1.0-3
Canna-3.7p3-7.EL4
Canna-devel-3.7p3-7.EL4
Canna-libs-3.7p3-7.EL4
ElectricFence-2.2.2-19
FreeWnn-1.10pl020-5
FreeWnn-devel-1.10pl020-5
FreeWnn-libs-1.10pl020-5
GConf-1.0.9-15
GConf2-2.8.1-1

if not, i suppose i need to run a diff of the package lists, extract the 
packages that need downgrading, and pass those to smart.  if anyone's 
been there before and knows the right incantation to diff, please share.

tia,
greg