docs: 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