(VAR X X1 X2 Y Z )
(RULES 
        active(dbl(0)) -> mark(0)
        active(dbl(s(X))) -> mark(s(s(dbl(X))))
        active(dbls(nil)) -> mark(nil)
        active(dbls(cons(X, Y))) -> mark(cons(dbl(X), dbls(Y)))
        active(sel(0, cons(X, Y))) -> mark(X)
        active(sel(s(X), cons(Y, Z))) -> mark(sel(X, Z))
        active(indx(nil, X)) -> mark(nil)
        active(indx(cons(X, Y), Z)) -> mark(cons(sel(X, Z), indx(Y, Z)))
        active(from(X)) -> mark(cons(X, from(s(X))))
        mark(dbl(X)) -> active(dbl(mark(X)))
        mark(0) -> active(0)
        mark(s(X)) -> active(s(X))
        mark(dbls(X)) -> active(dbls(mark(X)))
        mark(nil) -> active(nil)
        mark(cons(X1, X2)) -> active(cons(X1, X2))
        mark(sel(X1, X2)) -> active(sel(mark(X1), mark(X2)))
        mark(indx(X1, X2)) -> active(indx(mark(X1), X2))
        mark(from(X)) -> active(from(X))
        dbl(mark(X)) -> dbl(X)
        dbl(active(X)) -> dbl(X)
        s(mark(X)) -> s(X)
        s(active(X)) -> s(X)
        dbls(mark(X)) -> dbls(X)
        dbls(active(X)) -> dbls(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)
        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)
        indx(mark(X1), X2) -> indx(X1, X2)
        indx(X1, mark(X2)) -> indx(X1, X2)
        indx(active(X1), X2) -> indx(X1, X2)
        indx(X1, active(X2)) -> indx(X1, X2)
        from(mark(X)) -> from(X)
        from(active(X)) -> from(X)
        
)