A patch follows that includes an update to the documentation. The feature
is mentioned a couple of times in passing in 'Keeping Track'. I think it
deserves a (sub)section of its own and have inserted one called 'Following up
via direct email' in the 'Editing existing Problem Reports' section of 'The
GNATS User Tools' chapter. I have also corrected a couple of minor errors
that I ran across.
The regular expression used for matching the Subject line appears in the code
as
\\<(PR[ \t#/]?|([-A-Za-z0-9_+.]+)/)([0-9]+)
whereas the documentation has
\<(PR[ \t#/]?|[-\w+.]+/)[0-9]+