(VAR X X1 X2 Y Z )
(RULES 
        active(fst(0, Z)) -> mark(nil)
        active(fst(s(X), cons(Y, Z))) -> mark(cons(Y, fst(X, Z)))
        active(from(X)) -> mark(cons(X, from(s(X))))
        active(add(0, X)) -> mark(X)
        active(add(s(X), Y)) -> mark(s(add(X, Y)))
        active(len(nil)) -> mark(0)
        active(len(cons(X, Z))) -> mark(s(len(Z)))
        mark(fst(X1, X2)) -> active(fst(mark(X1), mark(X2)))
        mark(0) -> active(0)
        mark(nil) -> active(nil)
        mark(s(X)) -> active(s(X))
        mark(cons(X1, X2)) -> active(cons(mark(X1), X2))
        mark(from(X)) -> active(from(mark(X)))
        mark(add(X1, X2)) -> active(add(mark(X1), mark(X2)))
        mark(len(X)) -> active(len(mark(X)))
        fst(mark(X1), X2) -> fst(X1, X2)
        fst(X1, mark(X2)) -> fst(X1, X2)
        fst(active(X1), X2) -> fst(X1, X2)
        fst(X1, active(X2)) -> fst(X1, X2)
        s(mark(X)) -> s(X)
        s(active(X)) -> s(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)
        from(mark(X)) -> from(X)
        from(active(X)) -> from(X)
        add(mark(X1), X2) -> add(X1, X2)
        add(X1, mark(X2)) -> add(X1, X2)
        add(active(X1), X2) -> add(X1, X2)
        add(X1, active(X2)) -> add(X1, X2)
        len(mark(X)) -> len(X)
        len(active(X)) -> len(X)
        
)