In English law I think that this kind of clause has to be explicit and something that all parties sign up to - because the principle in English law is that part

In English law I think that this kind of clause has to be explicit and something that all parties sign up to - because the principle in English law is that parties can contract freely in any form that suits them. In other systems (and I think in the US - please correct me anyone who knows more!) you don't need an explicit clause to argue force majeure, you just need to get the court to agree that you are in extraordinary times.