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

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

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