(VAR X X1 X2 XS )
(RULES 
        active(zeros) -> mark(cons(0, zeros))
        active(tail(cons(X, XS))) -> mark(XS)
        mark(zeros) -> active(zeros)
        mark(cons(X1, X2)) -> active(cons(mark(X1), X2))
        mark(0) -> active(0)
        mark(tail(X)) -> active(tail(mark(X)))
        cons(mark(X1), X2) -> cons(X1, X2)
        cons(X1, mark(X2)) -> cons(X1, X2)
        cons(active(X1), X2) -> cons(X1, X2)
        cons(X1, active(X2)) -> cons(X1, X2)
        tail(mark(X)) -> tail(X)
        tail(active(X)) -> tail(X)
        
)