anchor.diff:  (defun zdd-remove-supersets-of% (zdd set)
     (ematch* (zdd set)
       ((_ nil) (leaf nil))
  -    (((leaf) _) zdd)
  +    (((sink) _) zdd)
       (((node var hi lo) (list* el remaining))
        (cond
          ((= var el) (zdd-node var
  
positive.code: (defun zdd-count (zdd)
    "Return the number of members of `zdd`."
    (ematch zdd
      ((leaf nil) 0)
      ((leaf t) 1)
      ((node _ hi lo) (+ (zdd-count hi)
                         (zdd-count lo)))))
  
  
negative.code:                   (tg:gc :full t :verbose t))
             args)))
  
  (defpattern sink (&optional content)
    `(structure leaf :content ,content))
  
  (defun never (val)
  