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

def get_unconnected_nodes(sentence_graph):
    """
    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
    """
    return [node for node in sentence_graph.nodes_iter()
            if sentence_graph.degree(node) == 0 and
            sentence_graph.number_of_nodes() > 1]