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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 async def createcsrf(self, csrfarg = '_csrf'):
 """ Create a anti-CSRF token in the session """

        await self.sessionstart()
        if not csrfarg in self.session.vars:
            self.session.vars[csrfarg] = uuid.uuid4().hex