(VAR X X1 X2 Y Z )
(RULES 
        active(f(X)) -> mark(cons(X, f(g(X))))
        active(g(0)) -> mark(s(0))
        active(g(s(X))) -> mark(s(s(g(X))))
        active(sel(0, cons(X, Y))) -> mark(X)
        active(sel(s(X), cons(Y, Z))) -> mark(sel(X, Z))
        active(f(X)) -> f(active(X))
        active(cons(X1, X2)) -> cons(active(X1), X2)
        active(g(X)) -> g(active(X))
        active(s(X)) -> s(active(X))
        active(sel(X1, X2)) -> sel(active(X1), X2)
        active(sel(X1, X2)) -> sel(X1, active(X2))
        f(mark(X)) -> mark(f(X))
        cons(mark(X1), X2) -> mark(cons(X1, X2))
        g(mark(X)) -> mark(g(X))
        s(mark(X)) -> mark(s(X))
        sel(mark(X1), X2) -> mark(sel(X1, X2))
        sel(X1, mark(X2)) -> mark(sel(X1, X2))
        proper(f(X)) -> f(proper(X))
        proper(cons(X1, X2)) -> cons(proper(X1), proper(X2))
        proper(g(X)) -> g(proper(X))
        proper(0) -> ok(0)
        proper(s(X)) -> s(proper(X))
        proper(sel(X1, X2)) -> sel(proper(X1), proper(X2))
        f(ok(X)) -> ok(f(X))
        cons(ok(X1), ok(X2)) -> ok(cons(X1, X2))
        g(ok(X)) -> ok(g(X))
        s(ok(X)) -> ok(s(X))
        sel(ok(X1), ok(X2)) -> ok(sel(X1, X2))
        top(mark(X)) -> top(proper(X))
        top(ok(X)) -> top(active(X))
        
)