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any expense incurred by the taxpayer, the amount of which is determined by the formula A × B where A is an expense that would be described in paragraph (g) if the reference to “March 21, 2013” in that paragraph were “2018” and that is not described in paragraph (g.3), and B is 100% if the expense is incurred before 2015, 80% if the expense is incurred in 2015, 60% if the expense is incurred in 2016, and 30% if the expense is incurred in 2017,