def _add_reciprocal_relations(triples_df):
    """Add reciprocal relations to the triples

    Parameters
    ----------

    triples_df : Dataframe
        Dataframe of triples

    Returns
    -------
    triples_df : Dataframe
        Dataframe of triples and their reciprocals
    """
    # create a copy of the original triples to add reciprocal relations
    df_reciprocal = triples_df.copy()

    # swap subjects and objects
    cols = list(df_reciprocal.columns)
    cols[0], cols[2] = cols[2], cols[0]
    df_reciprocal.columns = cols

    # add reciprocal relations
    df_reciprocal.iloc[:, 1] = df_reciprocal.iloc[:, 1] + "_reciprocal"

    # append to original triples
    triples_df = triples_df.append(df_reciprocal)
    return triples_df