WARNING: this is higly inefficient!
    Fix this via Issue #36.

    Returns
    -------
    all_layers : set or dict
        the set of all annotation layers used for annotating nodes in the given
        graph

def get_node_annotation_layers(docgraph):
    """
    WARNING: this is higly inefficient!
    Fix this via Issue #36.

    Returns
    -------
    all_layers : set or dict
        the set of all annotation layers used for annotating nodes in the given
        graph
    """
    all_layers = set()
    for node_id, node_attribs in docgraph.nodes_iter(data=True):
        for layer in node_attribs['layers']:
            all_layers.add(layer)
    return all_layers