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