Returns a single triple related to this node.

        :param key: Predicate of the triple
        :param lang: Language of the triple if applicable
        :rtype: Literal or BNode or URIRef

def get_single(self, key, lang=None):
        """ Returns a single triple related to this node.

        :param key: Predicate of the triple
        :param lang: Language of the triple if applicable
        :rtype: Literal or BNode or URIRef
        """
        if not isinstance(key, URIRef):
            key = URIRef(key)

        if lang is not None:
            default = None
            for o in self.graph.objects(self.asNode(), key):
                default = o
                if o.language == lang:
                    return o
            return default
        else:
            for o in self.graph.objects(self.asNode(), key):
                return o