(defun p+ (p1 p2)
  "Adds the two given polynomials together."
  (let ( (output '()) )
    (
      (mapcar #'(lambda (x) ; operates on successive elements of the first polynomial
       (concatenate 'list (list x) output)
      )  p1)
      (mapcar #'(lambda (y) ; does the same with the second polynomial
       (concatenate 'list (list y) output)
      ) p2)
    )
  )
)