(VAR V2 N M V1 X1 X2 X3 X)
(RULES 
active(U11(tt,V2)) -> mark(U12(isNat(V2)))
active(U12(tt)) -> mark(tt)
active(U21(tt)) -> mark(tt)
active(U31(tt,N)) -> mark(N)
active(U41(tt,M,N)) -> mark(U42(isNat(N),M,N))
active(U42(tt,M,N)) -> mark(s(plus(N,M)))
active(isNat(0)) -> mark(tt)
active(isNat(plus(V1,V2))) -> mark(U11(isNat(V1),V2))
active(isNat(s(V1))) -> mark(U21(isNat(V1)))
active(plus(N,0)) -> mark(U31(isNat(N),N))
active(plus(N,s(M))) -> mark(U41(isNat(M),M,N))
mark(U11(X1,X2)) -> active(U11(mark(X1),X2))
mark(tt) -> active(tt)
mark(U12(X)) -> active(U12(mark(X)))
mark(isNat(X)) -> active(isNat(X))
mark(U21(X)) -> active(U21(mark(X)))
mark(U31(X1,X2)) -> active(U31(mark(X1),X2))
mark(U41(X1,X2,X3)) -> active(U41(mark(X1),X2,X3))
mark(U42(X1,X2,X3)) -> active(U42(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) -> U11(X1,X2)
U11(X1,mark(X2)) -> U11(X1,X2)
U11(active(X1),X2) -> U11(X1,X2)
U11(X1,active(X2)) -> U11(X1,X2)
U12(mark(X)) -> U12(X)
U12(active(X)) -> U12(X)
isNat(mark(X)) -> isNat(X)
isNat(active(X)) -> isNat(X)
U21(mark(X)) -> U21(X)
U21(active(X)) -> U21(X)
U31(mark(X1),X2) -> U31(X1,X2)
U31(X1,mark(X2)) -> U31(X1,X2)
U31(active(X1),X2) -> U31(X1,X2)
U31(X1,active(X2)) -> U31(X1,X2)
U41(mark(X1),X2,X3) -> U41(X1,X2,X3)
U41(X1,mark(X2),X3) -> U41(X1,X2,X3)
U41(X1,X2,mark(X3)) -> U41(X1,X2,X3)
U41(active(X1),X2,X3) -> U41(X1,X2,X3)
U41(X1,active(X2),X3) -> U41(X1,X2,X3)
U41(X1,X2,active(X3)) -> U41(X1,X2,X3)
U42(mark(X1),X2,X3) -> U42(X1,X2,X3)
U42(X1,mark(X2),X3) -> U42(X1,X2,X3)
U42(X1,X2,mark(X3)) -> U42(X1,X2,X3)
U42(active(X1),X2,X3) -> U42(X1,X2,X3)
U42(X1,active(X2),X3) -> U42(X1,X2,X3)
U42(X1,X2,active(X3)) -> U42(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)
)