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Alist of pairs of server backends and search engines. The default associations are
          (nnimap . imap)
          (nntp . gmane)

A regexp to match newsgroups in the active file that should be skipped when searching all groups on a server.
The format specification to be used for lines in an nnir summary buffer. All the items from gnus-summary-line-format are available, along with three items unique to nnir summary buffers:
          %Z    Search retrieval score value (integer)
          %G    Article original full group name (string)
          %g    Article original short group name (string)

If nil (the default) this will use gnus-summary-line-format.

If non-nil, a function that retrieves article headers rather than using the gnus built-in function. This function takes an article list and group as arguments and populates the nntp-server-buffer with the retrieved headers. It should then return either 'nov or 'headers indicating the retrieved header format. Failure to retrieve headers should return nil.

If this variable is nil, or if the provided function returns nil for a search result, gnus-retrieve-headers will be called instead."