(defun tape-equilibrium (a)
  (let* ((vec (coerce a 'vector))
         (n (length vec))
         (first-part 0)
         (second-part (reduce #'+ vec))
         (min most-positive-fixnum))
    (loop for i from 0 below (1- n)
          do (progn
               (incf first-part i)
               (decf second-part i)
               (let ((difference (abs (- first-part second-part))))
                 (setf min (if (< difference min) difference min)))))
    min))