gnus: Loose Threads
3.9.1.1 Loose Threads
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‘gnus-summary-make-false-root’
If non-‘nil’, Gnus will gather all loose subtrees into one big tree
and create a dummy root at the top. (Wait a minute. Root at the
top? Yup.) Loose subtrees occur when the real root has expired,
or you’ve read or killed the root in a previous session.
When there is no real root of a thread, Gnus will have to fudge
something. This variable says what fudging method Gnus should use.
There are four possible values:
‘adopt’
Gnus will make the first of the orphaned articles the parent.
This parent will adopt all the other articles. The adopted
articles will be marked as such by pointy brackets (‘<>’)
instead of the standard square brackets (‘[]’). This is the
default method.
‘dummy’
Gnus will create a dummy summary line that will pretend to be
the parent. This dummy line does not correspond to any real
article, so selecting it will just select the first real
article after the dummy article.
‘gnus-summary-dummy-line-format’ is used to specify the format
of the dummy roots. It accepts only one format spec: ‘S’,
which is the subject of the article. 
Formatting
Variables. If you want all threads to have a dummy root,
even the non-gathered ones, set
‘gnus-summary-make-false-root-always’ to ‘t’.
‘empty’
Gnus won’t actually make any article the parent, but simply
leave the subject field of all orphans except the first empty.
(Actually, it will use ‘gnus-summary-same-subject’ as the
subject (Summary Buffer Format).)
‘none’
Don’t make any article parent at all. Just gather the threads
and display them after one another.
‘nil’
Don’t gather loose threads.
‘gnus-summary-gather-subject-limit’
Loose threads are gathered by comparing subjects of articles. If
this variable is ‘nil’, Gnus requires an exact match between the
subjects of the loose threads before gathering them into one big
super-thread. This might be too strict a requirement, what with
the presence of stupid newsreaders that chop off long subject
lines. If you think so, set this variable to, say, 20 to require
that only the first 20 characters of the subjects have to match.
If you set this variable to a really low number, you’ll find that
Gnus will gather everything in sight into one thread, which isn’t
very helpful.
If you set this variable to the special value ‘fuzzy’, Gnus will
use a fuzzy string comparison algorithm on the subjects (
Fuzzy Matching).
‘gnus-simplify-subject-fuzzy-regexp’
This can either be a regular expression or list of regular
expressions that match strings that will be removed from subjects
if fuzzy subject simplification is used.
‘gnus-simplify-ignored-prefixes’
If you set ‘gnus-summary-gather-subject-limit’ to something as low
as 10, you might consider setting this variable to something
sensible:
(setq gnus-simplify-ignored-prefixes
(concat
"\\`\\[?\\("
(mapconcat
'identity
'("looking"
"wanted" "followup" "summary\\( of\\)?"
"help" "query" "problem" "question"
"answer" "reference" "announce"
"How can I" "How to" "Comparison of"
;; ...
)
"\\|")
"\\)\\s *\\("
(mapconcat 'identity
'("for" "for reference" "with" "about")
"\\|")
"\\)?\\]?:?[ \t]*"))
All words that match this regexp will be removed before comparing
two subjects.
‘gnus-simplify-subject-functions’
If non-‘nil’, this variable overrides
‘gnus-summary-gather-subject-limit’. This variable should be a
list of functions to apply to the ‘Subject’ string iteratively to
arrive at the simplified version of the string.
Useful functions to put in this list include:
‘gnus-simplify-subject-re’
Strip the leading ‘Re:’.
‘gnus-simplify-subject-fuzzy’
Simplify fuzzily.
‘gnus-simplify-whitespace’
Remove excessive whitespace.
‘gnus-simplify-all-whitespace’
Remove all whitespace.
You may also write your own functions, of course.
‘gnus-summary-gather-exclude-subject’
Since loose thread gathering is done on subjects only, that might
lead to many false hits, especially with certain common subjects
like ‘’ and ‘(none)’. To make the situation slightly better, you
can use the regexp ‘gnus-summary-gather-exclude-subject’ to say
what subjects should be excluded from the gathering process.
The default is ‘^ *$\\|^(none)$’.
‘gnus-summary-thread-gathering-function’
Gnus gathers threads by looking at ‘Subject’ headers. This means
that totally unrelated articles may end up in the same “thread”,
which is confusing. An alternate approach is to look at all the
‘Message-ID’s in all the ‘References’ headers to find matches.
This will ensure that no gathered threads ever include unrelated
articles, but it also means that people who have posted with broken
newsreaders won’t be gathered properly. The choice is yours—plague
or cholera:
‘gnus-gather-threads-by-subject’
This function is the default gathering function and looks at
‘Subject’s exclusively.
‘gnus-gather-threads-by-references’
This function looks at ‘References’ headers exclusively.
If you want to test gathering by ‘References’, you could say
something like:
(setq gnus-summary-thread-gathering-function
'gnus-gather-threads-by-references)
Info Catalog
gnus: Customizing Threading
gnus: Filling In Threads