13. For the purposes of section 12, the undepreciated cost of property of one class, at any time, is equal to the capital cost to the operator of all the property of that class acquired before that time, less the aggregate of
(a) the total depreciation allowed the operator before that time in respect of that property;
(b) if the operator has disposed of property of that class before that time, the proceeds of disposition or the capital cost of the property, whichever is less; and
(c) each amount which has reduced the undepreciated cost of property of that class before that time under section 12 in respect of the disposition of property of the other class.