(VAR X X1 X2 Y Z )
(RULES 
        a__f(X) -> cons(mark(X), f(g(X)))
        a__g(0) -> s(0)
        a__g(s(X)) -> s(s(a__g(mark(X))))
        a__sel(0, cons(X, Y)) -> mark(X)
        a__sel(s(X), cons(Y, Z)) -> a__sel(mark(X), mark(Z))
        mark(f(X)) -> a__f(mark(X))
        mark(g(X)) -> a__g(mark(X))
        mark(sel(X1, X2)) -> a__sel(mark(X1), mark(X2))
        mark(cons(X1, X2)) -> cons(mark(X1), X2)
        mark(0) -> 0
        mark(s(X)) -> s(mark(X))
        a__f(X) -> f(X)
        a__g(X) -> g(X)
        a__sel(X1, X2) -> sel(X1, X2)
        
)