def condition_4i(eptm):
    """
    Return an index over the faces violating condition 4 i in Okuda et al 2013,
    that is edges (from the same face) sharing two vertices simultaneously.
    """
    num_srces = eptm.edge_df.groupby('face')['srce'].apply(lambda s: len(
        set(s)))
    num_sides = eptm.face_df['num_sides']
    return eptm.face_df[(num_srces != num_sides) | (num_sides < 3)].index
