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(in-package "ACL2")
(defun perm (x y)
(cond
((atom x) (atom y))
((member-equal (car x) y)
(perm (cdr x) (remove1-equal (car x) y)))
(t nil)))
(local
(defthm perm-reflexive
(perm x x)))
(local
(encapsulate
()
(local
(defthm perm-member
(implies (and (consp y)
(perm x y))
(member (car y) x))))
(local
(defthm perm-remove1
(implies (and (consp y) (perm x y))
(perm (remove1 (car y) x)
(cdr y)))))
(defthm perm-symmetric
(implies (perm x y)
(perm y x))
:hints (("Goal" :induct (perm y x))))))
(local
(encapsulate
()
(local
(defthm remove1-remove1
(equal (remove1 x (remove1 y z))
(remove1 y (remove1 x z)))))
(local
(defthm member-remove1
(implies (not (equal x y))
(iff (member x (remove1 y z))
(member x z)))))
(local
(defthm perm-remove1
(implies (perm x y)
(perm (remove1 e x)
(remove1 e y)))))
(local
(defthm perm-not-member
(implies (and (member e x)
(not (member e y)))
(not (perm x y)))
:hints (("Subgoal *1/3" :cases ((equal (car x) e))))))
(defthm perm-transitive
(implies (and (perm x y)
(perm y z))
(perm x z)))))
(defequiv perm)
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