(VAR M N X1 X2 X3 X)
(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)
)