assq

(assq <obj> <alist>) is defined as follows:

<alist> (for “association list”) must be a list of pairs. This procedure finds the first pair in <alist> whose car field is <obj> and returns that pair. If no pair in <alist> has <obj> as its car then #f (not the empty list) is returned. This procedure uses eq? to compare <obj> with the car fields of the pairs in <alist>.

Example:

[1] (assq 'c (list (list 'a 'b) (list 'c 'd) (list 'e 'f)))
(c d)
[2]