By contrast, within EC law, it is the effect or purpose of an agreement that determines whether or not it is subject to the law.

The Ninth Circuit reached that decision by applying the ancillary restraints doctrine, which provided an exception from the rule of reason whenever a restraint on trade was not ancillary to the main purpose of an agreement.

Apart from physical impossibility, frustration could be down to a contract becoming illegal to perform, for instance if war breaks out and the government bans trade to a belligerent country, or perhaps if the whole purpose of an agreement is destroyed by another event, like renting a room to watch a cancelled coronation parade.