"There is, however, no uncertainty as to the materials upon which the court must proceed. 'The data for decision are, on the one hand, the terms and construction of the contract, read in the light of the then existing circumstances, and on the other hand the events which have occurred' (Denny, Mott & Dickson Ltd. v. James B. Fraser & Co. Ltd, per Lord Wright). In the nature of things there is often no room for any elaborate inquiry. The court must act upon a general impression of what its rule requires. It is for that reason that special importance is necessarily attached to the occurrence of any unexpected event that, as it were, changes the face of things. But, even so, it is not hardship or inconvenience or material loss itself which calls the principle of frustration into play. There must be as well such a change in the significance of the obligation that the thing undertaken would, if performed, be a different thing from that contracted for."