(a)"accumulated loss" means so much of the loss of the predecessor firm or the proprietary concern or the amalgamating company or the demerged company, as the case may be, under the head 'profits and gains of business or profession" (not being a loss sustained in a speculation business) which such predecessor firm or the proprietary concern or amalgamating company or demerged company, would have been entitled to carry forward and set off under the provisions of section 72 if the reorganisation of business or amalgamation or demerger had not taken place;