def drop_two_sided_faces(eptm):
    """Removes all the two (or one?) sided faces from the epithelium

    Note that they are not collapsed, but simply eliminated
    Does not reindex
    """
    num_sides = eptm.edge_df.groupby('face').size()
    if num_sides.min() > 2:
        return
    two_sided = eptm.face_df[num_sides < 3].index
    edges = eptm.edge_df[eptm.edge_df['face'].isin(two_sided)].index
    eptm.edge_df.drop(edges, axis=0, inplace=True)
    eptm.face_df.drop(two_sided, axis=0, inplace=True)
