def tokenizer(self):
        """
        A property to link into IntentEngine's tokenizer.

        Warning: this is only for backwards compatiblility and should not be used if you
            intend on using domains.

        Return: the domains tokenizer from its IntentEngine
        """
        domain = 0
        if domain not in self.domains:
            self.register_domain(domain=domain)
        return self.domains[domain].tokenizer