(assign evalable-ld-printingp nil)

(defun mem (e x)
  (if (endp x)
    nil
    (if (equal e (car x))
      t
      (mem e (cdr x)))))

(defun eqv@ (a x eqv)
  (cond ((endp a) eqv)
        ((equal (car x) 'IF)
         (eqv@ (cdr a)
               (nth (car a) x)
               (if (equal (car a) 1) 'iff eqv)))
        ((equal (car x) 'AND)
         (eqv@ (cdr a)
               (nth (car a) x)
               (if (equal (car a) 1) 'iff eqv)))
        ((equal (car x) 'OR)
         (eqv@ (cdr a)
               (nth (car a) x)
               eqv))
        ((mem (car x) '(NOT IMPLIES IFF))
         (eqv@ (cdr a)
               (nth (car a) x)
               'iff))
        (t (eqv@ (cdr a)
                 (nth (car a) x)
                 'equal))))


(eqv@ '(2 1 1 2)
      '(implies (and (natp e)
                (mem e x))
           (natp (if (and (f e) (g x))
                     (h (f x))
                     x)))
      'IFF)

(eqv@ '(2 1 1 2 1)
      '(implies (and (natp e)
                (mem e x))
           (natp (if (and (f e) (g x))
                     (h (f x))
                     x)))
      'IFF)