+@node GroupLens
+@section GroupLens
+@cindex GroupLens
+
+GroupLens is a collaborative filtering system that helps you work
+together with other people to find the quality news articles out of the
+huge volume of news articles generated every day.
+
+To accomplish this the GroupLens system combines your opinions about
+articles you have already read with the opinions of others who have done
+likewise and gives you a personalized prediction for each unread news
+article. Think of GroupLens as a matchmaker. GroupLens watches how you
+rate articles, and finds other people that rate articles the same way.
+Once it has found for you some people you agree with it tells you, in
+the form of a prediction, what they thought of the article. You can use
+this prediction to help you decide whether or not you want to read the
+article.
+
+@menu
+* Using GroupLens:: How to make Gnus use GroupLens.
+* Rating Articles:: Letting GroupLens know how you rate articles.
+* Displaying Predictions:: Displaying predictions given by GroupLens.
+* GroupLens Variables:: Customizing GroupLens.
+@end menu
+
+
+@node Using GroupLens
+@subsection Using GroupLens
+
+To use GroupLens you must register a pseudonym with your local Better
+Bit Bureau (BBB).
+@samp{http://www.cs.umn.edu/Research/GroupLens/bbb.html} is the only
+better bit in town is at the moment.
+
+Once you have registered you'll need to set a couple of variables.
+
+@table @code
+
+@item gnus-use-grouplens
+@vindex gnus-use-grouplens
+Setting this variable to a non-@code{nil} value will make Gnus hook into
+all the relevant GroupLens functions.
+
+@item grouplens-pseudonym
+@vindex grouplens-pseudonym
+This variable should be set to the pseudonym you got when registering
+with the Better Bit Bureau.
+
+@item grouplens-newsgroups
+@vindex grouplens-newsgroups
+A list of groups that you want to get GroupLens predictions for.
+
+@end table
+
+Thats the minimum of what you need to get up and running with GroupLens.
+Once you've registered, GroupLens will start giving you scores for
+articles based on the average of what other people think. But, to get
+the real benefit of GroupLens you need to start rating articles
+yourself. Then the scores GroupLens gives you will be personalized for
+you, based on how the people you usually agree with have already rated.
+
+
+@node Rating Articles
+@subsection Rating Articles
+
+In GroupLens, an article is rated on a scale from 1 to 5, inclusive.
+Where 1 means something like this article is a waste of bandwidth and 5
+means that the article was really good. The basic question to ask
+yourself is, "on a scale from 1 to 5 would I like to see more articles
+like this one?"
+
+There are four ways to enter a rating for an article in GroupLens.
+
+@table @kbd
+
+@item r
+@kindex r (GroupLens)
+@findex bbb-summary-rate-article
+This function will prompt you for a rating on a scale of one to five.
+
+@item k
+@kindex k (GroupLens)
+@findex grouplens-score-thread
+This function will prompt you for a rating, and rate all the articles in
+the thread. This is really useful for some of those long running giant
+threads in rec.humor.
+
+@end table
+
+The next two commands, @kbd{n} and @kbd{,} take a numerical prefix to be
+the score of the article you're reading.
+
+@table @kbd
+
+@item 1-5 n
+@kindex n (GroupLens)
+@findex grouplens-next-unread-article
+Rate the article and go to the next unread article.
+
+@item 1-5 ,
+@kindex , (GroupLens)
+@findex grouplens-best-unread-article
+Rate the article and go to the next unread article with the highest score.
+
+@end table
+
+If you want to give the current article a score of 4 and then go to the
+next article, just type @kbd{4 n}.
+
+
+@node Displaying Predictions
+@subsection Displaying Predictions
+
+GroupLens makes a prediction for you about how much you will like a
+news article. The predictions from GroupLens are on a scale from 1 to
+5, where 1 is the worst and 5 is the best. You can use the predictions
+from GroupLens in one of three ways controlled by the variable
+@code{gnus-grouplens-override-scoring}.
+
+@vindex gnus-grouplens-override-scoring
+There are three ways to display predictions in grouplens. You may
+choose to have the GroupLens scores contribute to, or override the
+regular gnus scoring mechanism. override is the default; however, some
+people prefer to see the Gnus scores plus the grouplens scores. To get
+the separate scoring behavior you need to set
+@code{gnus-grouplens-override-scoring} to @code{'separate}. To have the
+GroupLens predictions combined with the grouplens scores set it to
+@code{'override} and to combine the scores set
+@code{gnus-grouplens-override-scoring} to @code{'combine}. When you use
+the combine option you will also want to set the values for
+@code{grouplens-prediction-offset} and
+@code{grouplens-score-scale-factor}.
+
+@vindex grouplens-prediction-display
+In either case, GroupLens gives you a few choices for how you would like
+to see your predictions displayed. The display of predictions is
+controlled by the @code{grouplens-prediction-display} variable.
+
+The following are legal values for that variable.
+
+@table @code
+@item prediction-spot
+The higher the prediction, the further to the right an @samp{*} is
+displayed.
+
+@item confidence-interval
+A numeric confidence interval.
+
+@item prediction-bar
+The higher the prediction, the longer the bar.
+
+@item confidence-bar
+Numerical confidence.
+
+@item confidence-spot
+The spot gets bigger with more confidence.
+
+@item prediction-num
+Plain-old numeric value.
+
+@item confidence-plus-minus
+Prediction +/i confidence.
+
+@end table
+
+
+@node GroupLens Variables
+@subsection GroupLens Variables
+
+@table @code
+
+@item gnus-summary-grouplens-line-format
+The summary line format used in summary buffers that are GroupLens
+enhanced. It accepts the same specs as the normal summary line format
+(@pxref{Summary Buffer Lines}). The default is
+@samp{%U%R%z%l%I%(%[%4L: %-20,20n%]%) %s\n}.
+
+@item grouplens-bbb-host
+Host running the bbbd server. @samp{grouplens.cs.umn.edu} is the
+default.
+
+@item grouplens-bbb-port
+Port of the host running the bbbd server. The default is 9000.
+
+@item grouplens-score-offset
+Offset the prediction by this value. In other words, subtract the
+prediction value by this number to arrive at the effective score. The
+default is 0.
+
+@item grouplens-score-scale-factor
+This variable allows the user to magnify the effect of GroupLens scores.
+The scale factor is applied after the offset. The default is 1.
+
+@end table
+
+
+@node Advanced Scoring
+@section Advanced Scoring
+
+Scoring on Subjects and From headers is nice enough, but what if you're
+really interested in what a person has to say only when she's talking
+about a particular subject? Or what about if you really don't want to
+read what person A has to say when she's following up to person B, but
+want to read what she says when she's following up to person C?
+
+By using advanced scoring rules you may create arbitrarily complex
+scoring patterns.
+
+@menu
+* Advanced Scoring Syntax:: A definition.
+* Advanced Scoring Examples:: What they look like.
+* Advanced Scoring Tips:: Getting the most out of it.
+@end menu
+
+
+@node Advanced Scoring Syntax
+@subsection Advanced Scoring Syntax
+
+Ordinary scoring rules have a string as the first element in the rule.
+Advanced scoring rules have a list as the first element. The second
+element is the score to be applied if the first element evaluated to a
+non-@code{nil} value.
+
+These lists may consist of three logical operators, one redirection
+operator, and various match operators.
+
+Logical operators:
+
+@table @code
+@item &
+@itemx and
+This logical operator will evaluate each of its arguments until it finds
+one that evaluates to @code{false}, and then it'll stop. If all arguments
+evaluate to @code{true} values, then this operator will return
+@code{true}.
+
+@item |
+@itemx or
+This logical operator will evaluate each of its arguments until it finds
+one that evaluates to @code{true}. If no arguments are @code{true},
+then this operator will return @code{false}.
+
+@item !
+@itemx not
+@itemx ¬
+This logical operator only takes a single argument. It returns the
+inverse of the value of its argument.
+
+@end table
+
+There is an @dfn{indirection operator} that will make its arguments
+apply to the ancestors of the current article being scored. For
+instance, @code{1-} will make score rules apply to the parent of the
+current article. @code{2-} will make score fules apply to the
+grandparent of the current article. Alternatively, you can write
+@code{^^}, where the number of @code{^}s (carets) say how far back into
+the ancestry you want to go.
+
+Finally, we have the match operators. These are the ones that do the
+real work. Match operators are header name strings followed by a match
+and a match type. A typical match operator looks like @samp{("from"
+"Lars Ingebrigtsen" s)}. The header names are the same as when using
+simple scoring, and the match types are also the same.
+
+
+@node Advanced Scoring Examples
+@subsection Advanced Scoring Examples
+
+Let's say you want to increase the score of articles written by Lars
+when he's talking about Gnus:
+
+@example
+((&
+ ("from" "Lars Ingebrigtsen")
+ ("subject" "Gnus"))
+ 1000)
+@end example
+
+Quite simple, huh?
+
+When he writes long articles, he sometimes has something nice to say:
+
+@example
+((&
+ ("from" "Lars Ingebrigtsen")
+ (|
+ ("subject" "Gnus")
+ ("lines" 100 >)))
+ 1000)
+@end example
+
+However, when he responds to things written by Reig Eigil Logge, you
+really don't want to read what he's written:
+
+@example
+((&
+ ("from" "Lars Ingebrigtsen")
+ (1- ("from" "Reig Eigir Logge")))
+ -100000)
+@end example
+
+Everybody that follows up Redmondo when he writes about disappearing
+socks should have their scores raised, but only when they talk about
+white socks. However, when Lars talks about socks, it's usually not
+very interesting:
+
+@example
+((&
+ (1-
+ (&
+ ("from" "redmondo@@.*no" r)
+ ("body" "disappearing.*socks" t)))
+ (! ("from" "Lars Ingebrigtsen"))
+ ("body" "white.*socks"))
+ 1000)
+@end example
+
+The possibilities are endless.
+
+
+@node Advanced Scoring Tips
+@subsection Advanced Scoring Tips
+
+The @code{&} and @code{|} logical operators do short-circuit logic.
+That is, they stop processing their arguments when it's clear what the
+result of the operation will be. For instance, if one of the arguments
+of an @code{&} evaluates to @code{false}, there's no point in evaluating
+the rest of the arguments. This means that you should put slow matches
+(@samp{body}, @code{header}) last and quick matches (@samp{from},
+@samp{subject}) first.
+
+The indirection arguments (@code{1-} and so on) will make their
+arguments work on previous generations of the thread. If you say
+something like:
+
+@example
+...
+(1-
+ (1-
+ ("from" "lars")))
+...
+@end example
+
+Then that means "score on the from header of the grandparent of the
+current article". An indirection is quite fast, but it's better to say:
+
+@example
+(1-
+ (&
+ ("from" "Lars")
+ ("subject" "Gnus")))
+@end example
+
+than it is to say:
+
+@example
+(&
+ (1- ("from" "Lars"))
+ (1- ("subject" "Gnus")))
+@end example
+
+
+@node Score Decays
+@section Score Decays
+@cindex score decays
+@cindex decays
+
+You may find that your scores have a tendency to grow without
+bounds, especially if you're using adaptive scoring. If scores get too
+big, they lose all meaning---they simply max out and it's difficult to
+use them in any sensible way.
+
+@vindex gnus-decay-scores
+@findex gnus-decay-score
+@vindex gnus-score-decay-function
+Gnus provides a mechanism for decaying scores to help with this problem.
+When score files are loaded and @code{gnus-decay-scores} is
+non-@code{nil}, Gnus will run the score files through the decaying
+mechanism thereby lowering the scores of all non-permanent score rules.
+The decay itself if performed by the @code{gnus-score-decay-function}
+function, which is @code{gnus-decay-score} by default. Here's the
+definition of that function:
+
+@lisp
+(defun gnus-decay-score (score)
+ (floor
+ (- score
+ (* (if (< score 0) 1 -1)
+ (min score
+ (max gnus-score-decay-constant
+ (* (abs score)
+ gnus-score-decay-scale)))))))
+@end lisp
+
+@vindex gnus-score-decay-scale
+@vindex gnus-score-decay-constant
+@code{gnus-score-decay-constant} is 3 by default and
+@code{gnus-score-decay-scale} is 0.05. This should cause the following:
+
+@enumerate
+@item
+Scores between -3 and 3 will be set to 0 when this function is called.
+
+@item
+Scores with magnitudes between 3 and 60 will be shrunk by 3.
+
+@item
+Scores with magnitudes greater than 60 will be shrunk by 5% of the
+score.
+@end enumerate
+
+If you don't like this decay function, write your own. It is called
+with the score to be decayed as its only parameter, and it should return
+the new score, which should be an integer.
+
+Gnus will try to decay scores once a day. If you haven't run Gnus for
+four days, Gnus will decay the scores four times, for instance.
+
+