§ 11. "That it is expedient to amend the provisions of the Income Tax Acts relating to the computation for the purposes of income tax of the profits or gains of a trade, profession or vocation so far as relates to the charge of income tax in respect of any such profits or gains for the year of assessment next after, and the year of assessment next but one after, the year of assessment in which the trade, profession or vocation was set up or commenced."