docs: def istoken(docgraph, node_id, namespace=None):
      """returns true, iff the given node ID belongs to a token node.
  
      Parameters
      ----------
      node_id : str
          the node to be checked
      namespace : str or None
          If a namespace is given, only look for tokens in the given namespace.
          Otherwise, look for tokens in the default namespace of the given
          document graph.
      """
      if namespace is None:
          namespace = docgraph.ns
      return namespace+':token' in docgraph.node[node_id]