;; Queue of pending operations. When empty, HEAD=TAIL=NIL. Otherwise,
;; TAIL holds the first (oldest) cons and HEAD holds the last (newest)
;; cons. Adding things to the queue is basically (setf (cdr head) (list
;; Queue of pending operations. When empty, HEAD=TAIL=NIL. Otherwise,
;; TAIL holds the first (oldest) cons and HEAD holds the last (newest)
;; cons. Adding things to the queue is basically (setf (cdr head) (list