rassoc

(rassoc value alist &key test key)

Searches an association list and returns the first pair whose cdr matches value, or nil if none matches. It is the mirror of assoc, which searches by car. The comparison is eql by default; the optional :test keyword takes a function designator to use a different comparison, and the optional :key keyword takes a selector function applied to each pair's cdr before the comparison. The returned pair shares structure with the alist.