def label_cumulative_occurrence(g, o_props):
    """label_cumulative_occurrence
    Calculates the cumulative occurrence values for vertices in a graph using
    the vertex occurrence values.

    Parameters
    ----------
    g : :obj:`graph_tool.Graph` 
        A graph.
    o_props : :obj:`dict` 
        Contains property maps of vertex occurrence values.

    Returns
    -------
    o_cumsum_props : :obj:`dict` 
        Contains property maps of type 'int' with vertex cumulative occurrence 
        values.
    """
    o_cumsum_props = {}
    for i, (step, o_prop) in enumerate(o_props.items()):
        o_cumsum_prop = g.new_vertex_property('int')
        if i == 0:
            o_cumsum_prop.a = o_prop.a
        else:
            o_cumsum_prop.a = (o_cumsum_props[step-1].a + o_prop.a)
        o_cumsum_props[step] = o_cumsum_prop
    return o_cumsum_props