(VAR M N X1 X2 X3 X)
(RULES 
a__U11(tt,M,N) -> a__U12(tt,M,N)
a__U12(tt,M,N) -> s(a__plus(mark(N),mark(M)))
a__plus(N,0) -> mark(N)
a__plus(N,s(M)) -> a__U11(tt,M,N)
mark(U11(X1,X2,X3)) -> a__U11(mark(X1),X2,X3)
mark(U12(X1,X2,X3)) -> a__U12(mark(X1),X2,X3)
mark(plus(X1,X2)) -> a__plus(mark(X1),mark(X2))
mark(tt) -> tt
mark(s(X)) -> s(mark(X))
mark(0) -> 0
a__U11(X1,X2,X3) -> U11(X1,X2,X3)
a__U12(X1,X2,X3) -> U12(X1,X2,X3)
a__plus(X1,X2) -> plus(X1,X2)
)