Smooth technique to clean up NOV after heavy deletion from outside

[Harry Putnam <reader@newsguy.com>]

I've just finished doing some rather massive (for a home setup)
deletions in the accumulated directories under News/agent/nntp

Somehow over the last dozen or more mnths, gnus has lost track of the
bulk of those files so that a gnus-agent-expire (even with `days' set
to 10) would barely put in a dent in the shear weight of files there.

I have no doubt that it was due to my own sloppy habits and endless
tinkering with not so much idea of what I'm doing.  Gnus cannot be
held accountable for loosing track of that stuff.

But any way, I have some truly massive NOV files left over and wasn't
sure if I should just run `gnus-agent-regenerate-group' and iterate
over the large number of groups, or if I should delete the exiting NOV
first.... or maybe something else altogether.

I seem to recall getting different results when using the similar
`nnml-generate-nov',  if I deleted the NOV first or not.

I've deleted everything the agent stored, that has been around longer
than 2 mnths but that still leaves some pretty hefty groups... so it
will be somewhat labor intensive regenerating the NOV in them.

Any ideas on best way to proceed.