(VAR M N X X1 X2 )
(RULES 
        a__and(tt, X) -> mark(X)
        a__plus(N, 0) -> mark(N)
        a__plus(N, s(M)) -> s(a__plus(mark(N), mark(M)))
        a__x(N, 0) -> 0
        a__x(N, s(M)) -> a__plus(a__x(mark(N), mark(M)), mark(N))
        mark(and(X1, X2)) -> a__and(mark(X1), X2)
        mark(plus(X1, X2)) -> a__plus(mark(X1), mark(X2))
        mark(x(X1, X2)) -> a__x(mark(X1), mark(X2))
        mark(tt) -> tt
        mark(0) -> 0
        mark(s(X)) -> s(mark(X))
        a__and(X1, X2) -> and(X1, X2)
        a__plus(X1, X2) -> plus(X1, X2)
        a__x(X1, X2) -> x(X1, X2)
        
)