Optional Backend Functions
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`(nnchoke-retrieve-groups GROUPS &optional SERVER)'
     GROUPS is a list of groups, and this function should request data
     on all those groups.  How it does it is of no concern to Gnus, but
     it should attempt to do this in a speedy fashion.

     The return value of this function can be either `active' or
     `group', which says what the format of the result data is.  The
     former is in the same format as the data from
     `nnchoke-request-list', while the latter is a buffer full of lines
     in the same format as `nnchoke-request-group' gives.

          group-buffer = *active-line / *group-status

`(nnchoke-request-update-info GROUP INFO &optional SERVER)'
     A Gnus group info (see Group Info.) is handed to the backend
     for alterations.  This comes in handy if the backend really
     carries all the information (as is the case with virtual an imap
     groups).  This function may alter the info in any manner it sees
     fit, and should return the (altered) group info.  This function
     may alter the group info destructively, so no copying is needed
     before boogeying.

     There should be no result data from this function.

`(nnchoke-request-type GROUP &optional ARTICLE)'
     When the user issues commands for "sending news" (`F' in the
     summary buffer, for instance), Gnus has to know whether the
     article the user is following up is news or mail.  This function
     should return `news' if ARTICLE in GROUP is news, `mail' if it is
     mail and `unknown' if the type can't be decided.  (The ARTICLE
     parameter is necessary in `nnvirtual' groups which might very well
     combine mail groups and news groups.)

     There should be no result data from this function.

`(nnchoke-request-update-mark GROUP ARTICLE MARK)'
     If the user tries to set a mark that the backend doesn't like, this
     function may change the mark.  Gnus will use whatever this function
     returns as the mark for ARTICLE instead of the original MARK.  If
     the backend doesn't care, it must return the original MARK, and
     not `nil' or any other type of garbage.

     The only use for this that I can see is what `nnvirtual' does with
     it--if a component group is auto-expirable, marking an article as
     read in the virtual group should result in the article being
     marked as expirable.

     There should be no result data from this function.

`(nnchoke-request-scan &optional GROUP SERVER)'
     This function may be called at any time (by Gnus or anything else)
     to request that the backend check for incoming articles, in one
     way or another.  A mail backend will typically read the spool file
     or query the POP server when this function is invoked.  The GROUP
     doesn't have to be heeded--if the backend decides that it is too
     much work just scanning for a single group, it may do a total scan
     of all groups.  It would be nice, however, to keep things local if
     that's practical.

     There should be no result data from this function.

`(nnchoke-request-asynchronous GROUP &optional SERVER ARTICLES)'
     This is a request to fetch articles asynchronously later.
     ARTICLES is an alist of (ARTICLE-NUMBER LINE-NUMBER).  One would
     generally expect that if one later fetches article number 4, for
     instance, some sort of asynchronous fetching of the articles after
     4 (which might be 5, 6, 7 or 11, 3, 909 depending on the order in
     that alist) would be fetched asynchronously, but that is left up
     to the backend.  Gnus doesn't care.

     There should be no result data from this function.

`(nnchoke-request-group-description GROUP &optional SERVER)'
     The result data from this function should be a description of
     GROUP.

          description-line = name <TAB> description eol
          name             = <string>
          description      = <text>

`(nnchoke-request-list-newsgroups &optional SERVER)'
     The result data from this function should be the description of all
     groups available on the server.

          description-buffer = *description-line

`(nnchoke-request-newgroups DATE &optional SERVER)'
     The result data from this function should be all groups that were
     created after `date', which is in normal human-readable date
     format.  The data should be in the active buffer format.

`(nnchoke-request-create-group GROUP &optional SERVER)'
     This function should create an empty group with name GROUP.

     There should be no return data.

`(nnchoke-request-expire-articles ARTICLES &optional GROUP SERVER FORCE)'
     This function should run the expiry process on all articles in the
     ARTICLES range (which is currently a simple list of article
     numbers.)  It is left up to the backend to decide how old articles
     should be before they are removed by this function.  If FORCE is
     non-`nil', all ARTICLES should be deleted, no matter how new they
     are.

     This function should return a list of articles that it did not/was
     not able to delete.

     There should be no result data returned.

`(nnchoke-request-move-article ARTICLE GROUP SERVER ACCEPT-FORM'
     &optional LAST)

     This function should move ARTICLE (which is a number) from GROUP
     by calling ACCEPT-FORM.

     This function should ready the article in question for moving by
     removing any header lines it has added to the article, and
     generally should "tidy up" the article.  Then it should `eval'
     ACCEPT-FORM in the buffer where the "tidy" article is.  This will
     do the actual copying.  If this `eval' returns a non-`nil' value,
     the article should be removed.

     If LAST is `nil', that means that there is a high likelihood that
     there will be more requests issued shortly, so that allows some
     optimizations.

     The function should return a cons where the car is the group name
     and the cdr is the article number that the article was entered as.

     There should be no data returned.

`(nnchoke-request-accept-article GROUP &optional SERVER LAST)'
     This function takes the current buffer and inserts it into GROUP.
     If LAST in `nil', that means that there will be more calls to this
     function in short order.

     The function should return a cons where the car is the group name
     and the cdr is the article number that the article was entered as.

     There should be no data returned.

`(nnchoke-request-replace-article ARTICLE GROUP BUFFER)'
     This function should remove ARTICLE (which is a number) from GROUP
     and insert BUFFER there instead.

     There should be no data returned.

`(nnchoke-request-delete-group GROUP FORCE &optional SERVER)'
     This function should delete GROUP.  If FORCE, it should really
     delete all the articles in the group, and then delete the group
     itself.  (If there is such a thing as "the group itself".)

     There should be no data returned.

`(nnchoke-request-rename-group GROUP NEW-NAME &optional SERVER)'
     This function should rename GROUP into NEW-NAME.  All articles
     that are in GROUP should move to NEW-NAME.

     There should be no data returned.