427. Specific provision for the occurrence or consequences of prevention and hindrance.

Certain propositions may be drawn from the authorities on provisions in common use. Clauses covering 'hindrance' of performance by specified events apply if such an event occurs giving rise to obstacles which it would be very difficult to overcome1. Clauses covering 'prevention' or 'prohibition' by specified events have been said to require proof of legal or physical impossibility of performance2. The better view seems, however, to be that commercial impracticability may also constitute prevention or prohibition3.

Under a clause applying generally, as in the case of a prohibition of the export of goods of the contract description, the seller need