But there isnt.
When I mirandize folks, I inform them that they can stop answering questions or ask for a lawyer at any time. That isn't a requirement of the Supreme Court's ruling in Miranda.
I also ask them at the end of the miranda warning, "Do you want to answer questions at this time without an attorney present?"
The only things that are required by Miranda are that you inform them that they don't have to answer questions as anything they say may be used against them in court, and they have the right to have legal counsel present during questioning.