418.5. In this chapter, cumulative Canadian oil and gas property expense of a taxpayer at any time in a taxation year means the amount by which the aggregate(a) of the Canadian oil and gas property expense incurred by the taxpayer before that time,
(a.1) of the aggregate of all amounts determined under paragraph c of section 418.31.1 in respect of the taxpayer for a taxation year ending before that time,
(b) of all amounts determined under section 418.12 in respect of the taxpayer for any taxation year ending before that time,
(c) of all amounts contemplated in paragraph b or c of section 418.6 that, in accordance with the evidence submitted by the taxpayer, have become a bad debt before that time, and
(d) of such part of an amount contemplated in paragraph e of section 418.6 as has been repaid by him before that time pursuant to a legal obligation to repay all or any part of that amount exceeds the aggregate described in section 418.6.