anchor.diff:  ;;;
   ;;; (bench  (list 1 2 3 4 5 6 7 8)) -> 25.77 nanoseconds
   ;;; (bench (ulist 1 2 3 4 5 6 7 8)) -> 38.18 nanoseconds
  -
  -(declaim (hash-table *root-table*))
  -(defvar *root-table* (make-hash-table)
  -  "The table of all uconses whose cdr is NIL.")
   
   (defstruct (ucons
               (:constructor make-fresh-ucons (car cdr))
  
positive.code:           (let ((hash-table (alist-hash-table (utable cdr) :size 32)))
              (setf (gethash car hash-table) ucons)
              (setf (utable cdr) hash-table))))))
  
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;
  
negative.code:                   finally (return (ucons-list car cdr)))
              (ucons-hash car cdr)))))
  
  ;;; Called if the UTABLE of CDR is a hash table
  (defun ucons-hash (car cdr)
    (declare (ucons cdr))
  