(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)))
        active(cons(X1, X2)) -> cons(active(X1), X2)
        active(fst(X1, X2)) -> fst(active(X1), X2)
        active(fst(X1, X2)) -> fst(X1, active(X2))
        active(from(X)) -> from(active(X))
        active(add(X1, X2)) -> add(active(X1), X2)
        active(add(X1, X2)) -> add(X1, active(X2))
        active(len(X)) -> len(active(X))
        cons(mark(X1), X2) -> mark(cons(X1, X2))
        fst(mark(X1), X2) -> mark(fst(X1, X2))
        fst(X1, mark(X2)) -> mark(fst(X1, X2))
        from(mark(X)) -> mark(from(X))
        add(mark(X1), X2) -> mark(add(X1, X2))
        add(X1, mark(X2)) -> mark(add(X1, X2))
        len(mark(X)) -> mark(len(X))
        proper(0) -> ok(0)
        proper(s(X)) -> s(proper(X))
        proper(nil) -> ok(nil)
        proper(cons(X1, X2)) -> cons(proper(X1), proper(X2))
        proper(fst(X1, X2)) -> fst(proper(X1), proper(X2))
        proper(from(X)) -> from(proper(X))
        proper(add(X1, X2)) -> add(proper(X1), proper(X2))
        proper(len(X)) -> len(proper(X))
        s(ok(X)) -> ok(s(X))
        cons(ok(X1), ok(X2)) -> ok(cons(X1, X2))
        fst(ok(X1), ok(X2)) -> ok(fst(X1, X2))
        from(ok(X)) -> ok(from(X))
        add(ok(X1), ok(X2)) -> ok(add(X1, X2))
        len(ok(X)) -> ok(len(X))
        top(mark(X)) -> top(proper(X))
        top(ok(X)) -> top(active(X))
        
)