The function ring-push
adds any Lisp object as an element of the ring before the "insertion position", which means that a following ring-pop will return it. If the ring is full, that is the number of elements in the ring is the same as its size (see make-ring), then ring-push
first removes the element after the insertion point.
Once it finished modifying the ring, if ring-push
removed an element and there is a delete-function (see make-ring), then ring-push
calls delete-function on the element that it removes.
LispWorks User Guide and Reference Manual - 13 Feb 2015