(defun max-double-slice-sum (a)
  (let* ((vec (coerce a 'vector))
         (size (length vec)))
    (if (< size 3)
        0
        (let ((p1 (make-array size :initial-element 0))
              (p2 (make-array size :initial-element 0)))
          (setf (aref p1 1) 0)
          (setf (aref p2 (- size 2)) 0)
          (loop for i from 2 below (1- size)
                do (progn
                     (setf (aref p1 i) (max 0 (+ (aref p1 (1- i)) (aref vec (1- i)))))
                     (setf (aref p2 (- size i 1)) (max 0 (+ (aref p2 (- size i)) (aref vec (- size i)))))))
          (let ((sum (+ (aref p1 1) (aref p2 1))))
            (loop for i from 1 below (1- size)
                  do (setf sum (max sum (+ (aref p1 i) (aref p2 i)))))
            sum)))))