(VAR M N X X1 X2 X3 )
(RULES 
        active(U11(tt, M, N)) -> mark(U12(tt, M, N))
        active(U12(tt, M, N)) -> mark(s(plus(N, M)))
        active(plus(N, 0)) -> mark(N)
        active(plus(N, s(M))) -> mark(U11(tt, M, N))
        mark(U11(X1, X2, X3)) -> active(U11(mark(X1), X2, X3))
        mark(tt) -> active(tt)
        mark(U12(X1, X2, X3)) -> active(U12(mark(X1), X2, X3))
        mark(s(X)) -> active(s(mark(X)))
        mark(plus(X1, X2)) -> active(plus(mark(X1), mark(X2)))
        mark(0) -> active(0)
        U11(mark(X1), X2, X3) -> U11(X1, X2, X3)
        U11(X1, mark(X2), X3) -> U11(X1, X2, X3)
        U11(X1, X2, mark(X3)) -> U11(X1, X2, X3)
        U11(active(X1), X2, X3) -> U11(X1, X2, X3)
        U11(X1, active(X2), X3) -> U11(X1, X2, X3)
        U11(X1, X2, active(X3)) -> U11(X1, X2, X3)
        U12(mark(X1), X2, X3) -> U12(X1, X2, X3)
        U12(X1, mark(X2), X3) -> U12(X1, X2, X3)
        U12(X1, X2, mark(X3)) -> U12(X1, X2, X3)
        U12(active(X1), X2, X3) -> U12(X1, X2, X3)
        U12(X1, active(X2), X3) -> U12(X1, X2, X3)
        U12(X1, X2, active(X3)) -> U12(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)
        
)