Create the pandas.Series containing the side labels that correspond
    to each pathway, based on the user-specified gene signature definition.

def _pathway_side_information(pathway_positive_series,
                              pathway_negative_series,
                              index):
    """Create the pandas.Series containing the side labels that correspond
    to each pathway, based on the user-specified gene signature definition.
    """
    positive_series_label = pd.Series(["pos"] * len(pathway_positive_series))
    negative_series_label = pd.Series(["neg"] * len(pathway_negative_series))
    side_information = positive_series_label.append(
        negative_series_label)
    side_information.index = index
    side_information.name = "side"
    return side_information