25. In the case contemplated in section 22 a partner may deduct, in computing his income for the 1972 taxation year and the following years, to the extent that he has not deducted it for a previous year, an amount that is the aggregate:(a) of the lesser of the undepreciated capital cost to that partner of property of that class on 31 December 1971 or of the excess of the capital cost to such partner of all the property of that class contemplated in the said section, over the portion of the capital cost of that class to the partnership corresponding to the percentage of interest of that partner in property of that class; and
(b) of an amount equal to the excess of the undepreciated capital cost of property of that class to such partner on 31 December 1971 less the amount contemplated in paragraph a for the property of that class, over the portion of the undepreciated capital cost of property of that class to the partnership equal to the percentage of interest of that partner in such property.