(mod (p)

  (include *standard-cl-23*)

  (defun fromMaybe (v (@ m (content))) (if m content v))
  (defun mapMaybe (f (@ m (content))) (if m (list (a f (list content))) ()))

  (fromMaybe 0 (mapMaybe (lambda ((@ pt (x y))) (+ x y)) p))
  )