(VAR X XS X1 X2)
(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))
)