def build_features_ba_less_pa_over_cl(df):
    """
    This function takes the wrangled data DataFrame as its argument,
    calculates the ratio of ((bill amount−pay amount)/credit limit)
    for each of the six pairs of BILL_AMT and PAY_AMT features, and 
    returns a new DataFrame that includes the six new 
    ((bill amount−pay amount)/credit limit) features, 
    in addition to the original wrangled data DataFrame.
    """

    bill_amount_column_list = ['BILL_AMT1',
                               'BILL_AMT2',
                               'BILL_AMT3',
                               'BILL_AMT4',
                               'BILL_AMT5',
                               'BILL_AMT6']

    pay_amount_column_list = ['PAY_AMT1',
                              'PAY_AMT2',
                              'PAY_AMT3',
                              'PAY_AMT4',
                              'PAY_AMT5',
                              'PAY_AMT6'] 
    
    for i, (ba, pa) in enumerate(zip(bill_amount_column_list, 
                               pay_amount_column_list), 
                           1):

        new_column_name = 'ba_less_pa_over_cl_' + str(i)
        
        df[new_column_name] = \
            (df[ba] -df[pa])/ df['LIMIT_BAL']

    return df