Some Gnus variables are predicate specifiers. This is a special form that allows flexible specification of predicates without having to type all that much.
These specifiers are lists consisting of functions, symbols and lists.
Here's an example:
(or gnus-article-unseen-p
gnus-article-unread-p)
The available symbols are or, and
and not. The functions all take one parameter.
Internally, Gnus
calls gnus-make-predicate on these specifiers to
create a function that can be called. This input parameter to
this function will be passed along to all the functions in the
predicate specifier.