anchor.diff:                                    (zdd-join% lo-a lo-b)))))))
   
   (defun zdd-join (&rest zdds)
  +  "Return the relational join of ZDDs: {α ∪ β | α ∈ Z₁ and β ∈ Z₂}."
     (if zdds
       (reduce #'zdd-join% zdds)
       (sink nil)))
  
positive.code:                                   (zdd-union% hi-a hi-b)
                                    (zdd-union% lo-a lo-b)))))))
  (defun zdd-union (&rest zdds)
    (if zdds
      (reduce #'zdd-union% zdds)
      (sink nil)))
  
negative.code:        ((> var el) (zdd-keep-avoiders-of% zdd remaining))))))
  
  (defun zdd-keep-avoiders-of (zdd set)
    "Return a ZDD of members of `zdd` avoiding `set`: {α | α ∈ Z and α ∩ S = ø}."
    (zdd-keep-avoiders-of% zdd (sort set #'<)))
  
  
  