The following:

    ;; If we can find an association
    ((assq v s) => 
    ;; apply this lambda to it i.e. apply f (assq v s)
     (lambda (a) f a))
    (else v))

Is the same as:

;; Bind a variable `a` to the result of finding the association of v in s
(let ((a (assq v s)))
    ;; if we can find the association `a`, apply f to it.
    (a (f a))
    (else v)))