def get_zero_features_in_matrix(df):
    """
    Given a pandas DataFrame loaded from a matrix_X*.csv file,
    return all columns (features) where the values are always zero.
    """
    zero_events = []
    for col in df:
        if not df[col].any():
            # col.split(".", 1)[1] for getting event name
            zero_events.append(col)
    return zero_events