def trim(table):
    """ Keep only the columns prescribed by the Ketos annotation format.

        Args:
            table: pandas DataFrame
                Annotation table. 

        Returns:
            table: pandas DataFrame
                Annotation table, after removal of columns.
    """
    keep_cols = ['filename', 'label', 'start', 'end', 'freq_min', 'freq_max']
    drop_cols = [x for x in table.columns.values if x not in keep_cols]
    table = table.drop(drop_cols, axis=1)
    return table