(VAR M N V1 V2 X X1 X2 X3 )
(RULES 
        active(U11(tt, N)) -> mark(N)
        active(U21(tt, M, N)) -> mark(s(plus(N, M)))
        active(and(tt, X)) -> mark(X)
        active(isNat(0)) -> mark(tt)
        active(isNat(plus(V1, V2))) -> mark(and(isNat(V1), isNat(V2)))
        active(isNat(s(V1))) -> mark(isNat(V1))
        active(plus(N, 0)) -> mark(U11(isNat(N), N))
        active(plus(N, s(M))) -> mark(U21(and(isNat(M), isNat(N)), M, N))
        mark(U11(X1, X2)) -> active(U11(mark(X1), X2))
        mark(tt) -> active(tt)
        mark(U21(X1, X2, X3)) -> active(U21(mark(X1), X2, X3))
        mark(s(X)) -> active(s(mark(X)))
        mark(plus(X1, X2)) -> active(plus(mark(X1), mark(X2)))
        mark(and(X1, X2)) -> active(and(mark(X1), X2))
        mark(isNat(X)) -> active(isNat(X))
        mark(0) -> active(0)
        U11(mark(X1), X2) -> U11(X1, X2)
        U11(X1, mark(X2)) -> U11(X1, X2)
        U11(active(X1), X2) -> U11(X1, X2)
        U11(X1, active(X2)) -> U11(X1, X2)
        U21(mark(X1), X2, X3) -> U21(X1, X2, X3)
        U21(X1, mark(X2), X3) -> U21(X1, X2, X3)
        U21(X1, X2, mark(X3)) -> U21(X1, X2, X3)
        U21(active(X1), X2, X3) -> U21(X1, X2, X3)
        U21(X1, active(X2), X3) -> U21(X1, X2, X3)
        U21(X1, X2, active(X3)) -> U21(X1, X2, X3)
        s(mark(X)) -> s(X)
        s(active(X)) -> s(X)
        plus(mark(X1), X2) -> plus(X1, X2)
        plus(X1, mark(X2)) -> plus(X1, X2)
        plus(active(X1), X2) -> plus(X1, X2)
        plus(X1, active(X2)) -> plus(X1, X2)
        and(mark(X1), X2) -> and(X1, X2)
        and(X1, mark(X2)) -> and(X1, X2)
        and(active(X1), X2) -> and(X1, X2)
        and(X1, active(X2)) -> and(X1, X2)
        isNat(mark(X)) -> isNat(X)
        isNat(active(X)) -> isNat(X)
        
)