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