Making a branch to patch a previous release:

 cvs checkout -r Release-3-1-3 ProofGeneral
 cd ProofGeneral
 cvs tag -b Release-3-1-branch 
 # Drop this repo, has sticky tag for 3-1-3.
 cd .. ; cvs release -d ProofGeneral
 # Get new one
 cvs checkout -r Release-3-1-branch ProofGeneral
 cd ProofGeneral
 patch  ... blah ...
 cvs commit  ... blah ..

Then make release as ~proofgen:

 mkdir oldbranch
 cd oldbranch
 cvs checkout -r Release-3-1-branch ProofGeneral
 make devel.releaseall VERSION=3.1 FULLVERSION=3.1.99

NB: See warning in Makefile.devel about this (it will
downgrade web pages).
 
Then perhaps merge in branch changes into main stream:

 cd projects/proofgen/ProofGeneral
 cvs update -jRelease-3-1-branch

NB: this merging will always create conficts with 
$Id$ tags in source. (Is there a way to avoid this?)