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
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