code: def get_unconnected_nodes(sentence_graph):
      
      return [node for node in sentence_graph.nodes_iter()
              if sentence_graph.degree(node) == 0 and
              sentence_graph.number_of_nodes() > 1]
docs: Takes a TigerSentenceGraph and returns a list of node IDs of
      unconnected nodes.
  
      A node is unconnected, if it doesn't have any in- or outgoing edges.
      A node is NOT considered unconnected, if the graph only consists of
      that particular node.
  
      Parameters
      ----------
      sentence_graph : TigerSentenceGraph
          a directed graph representing one syntax annotated sentence from
          a TigerXML file
  
      Returns
      -------
      unconnected_node_ids : list of str
          a list of node IDs of unconnected nodes