(defun dominator (a)
  (let ((vec (coerce a 'vector))
        (size 0) (value 0) (index 0))
    (loop for k from 0 below (length vec)
          for v = (aref vec k)
          do (cond
               ((zerop size) (incf size) (setf value v) (setf index k))
               ((/= value v) (decf size))
               (t (incf size))))
    (let ((candidate (if (> size 0) value -1))
          (count 0))
      (loop for v across vec do (when (= v candidate) (incf count)))
      (when (<= count (/ (length vec) 2))
        (setf index -1))
      index)))