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• Ubdd-constructors

Q-iff

(q-iff a b &rest args) constructs a UBDD representing an equality/iff-equivalence.

In the logic, for instance,

(q-iff x1 x2 x3 x4) = (q-and (q-binary-iff x1 x2)
                             (q-binary-iff x1 x3)
                             (q-binary-iff x1 x4))

We do not see how to use short-circuiting to improve upon (q-binary-iff x y), since the answer depends upon both inputs in all cases. But when we implement the multiple-input q-iff macro, we can at least take advantage of the short-circuiting provided by q-and. We also reorder the inputs so that cheap ones come first, hoping that we can avoid evaluating expensive arguments.

Definitions and Theorems

Function: q-binary-iff

(defun q-binary-iff (x y)
  (declare (xargs :guard t))
  (cond ((atom x) (if x y (q-not y)))
        ((atom y) (if y x (q-not x)))
        ((hons-equal x y) t)
        (t (qcons (q-binary-iff (car x) (car y))
                  (q-binary-iff (cdr x) (cdr y))))))

Function: q-binary-iff-memoize-condition

(defun q-binary-iff-memoize-condition (x y)
  (declare (ignorable x y)
           (xargs :guard 't))
  (and (consp x) (consp y)))

Function: q-iff-macro-guts

(defun q-iff-macro-guts (a x)
  (if (consp x)
      (cons (cons 'q-binary-iff
                  (cons a (cons (car x) 'nil)))
            (q-iff-macro-guts a (cdr x)))
    nil))

Function: q-iff-macro-fn

(defun q-iff-macro-fn (args)
 (if (equal (len args) 2)
     (cons 'prog2$
           (cons '(last-chance-wash-memory)
                 (cons (cons 'q-binary-iff
                             (cons (car args)
                                   (cons (cadr args) 'nil)))
                       'nil)))
  (mv-let (cheap expensive)
          (cheap-and-expensive-arguments args)
   (let ((reordered-args (append cheap expensive)))
    (cons
     'let
     (cons
      (cons (cons 'a-for-q-iff-do-not-use-elsewhere
                  (cons (car reordered-args) 'nil))
            'nil)
      (cons
       (cons
        'prog2$
        (cons
         '(last-chance-wash-memory)
         (cons
           (cons 'q-and
                 (q-iff-macro-guts 'a-for-q-iff-do-not-use-elsewhere
                                   (cdr reordered-args)))
           'nil)))
       'nil)))))))

Theorem: ubddp-of-q-iff

(defthm ubddp-of-q-iff
  (implies (and (force (ubddp x))
                (force (ubddp y)))
           (equal (ubddp (q-iff x y)) t)))

Theorem: eval-bdd-of-q-iff

(defthm eval-bdd-of-q-iff
  (equal (eval-bdd (q-iff x y) values)
         (iff (eval-bdd x values)
              (eval-bdd y values))))

Theorem: canonicalize-q-iff

(defthm canonicalize-q-iff
  (implies (and (force (ubddp x))
                (force (ubddp y)))
           (equal (q-iff x y)
                  (q-ite x y (q-ite y nil t)))))