The function ring-ref
returns or sets the element at index places before the insertion point in the ring ring.
index must be a non-negative integer smaller than the number of elements in the ring, otherwise an error is signaled. index 0 returns or sets the element object just before the insertion point, and a larger index goes "back" (in the same direction as ring-pop and rotate-ring).
The setf
function replaces the element in the ring with the new element object without affecting the ring otherwise (in particular it does not call delete-function).
LispWorks User Guide and Reference Manual - 20 Sep 2017