Common lisp :KEY parameter use

Question

The :KEY parameter is included in some functions that ship with Common Lisp. All of the descriptions that I have found of them are unhelpful, and :KEY

Answer 1

The :key argument is a function of one parameter; it is applied to each element of the sequence to generate the value used for testing. If omitted, the identity function is used.

Here's an example from the CLHS:

(member 2 '((1 . 2) (3 . 4)) :test-not #'= :key #'cdr) =>  ((3 . 4))