(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))
        mark(f(X)) -> active(f(mark(X)))
        mark(cons(X1, X2)) -> active(cons(mark(X1), X2))
        mark(g(X)) -> active(g(mark(X)))
        mark(0) -> active(0)
        mark(s(X)) -> active(s(mark(X)))
        mark(sel(X1, X2)) -> active(sel(mark(X1), mark(X2)))
        f(mark(X)) -> f(X)
        f(active(X)) -> f(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)
        g(mark(X)) -> g(X)
        g(active(X)) -> g(X)
        s(mark(X)) -> s(X)
        s(active(X)) -> s(X)
        sel(mark(X1), X2) -> sel(X1, X2)
        sel(X1, mark(X2)) -> sel(X1, X2)
        sel(active(X1), X2) -> sel(X1, X2)
        sel(X1, active(X2)) -> sel(X1, X2)
        
)