law: Indian Contract Act, 1872 
  33. Enforcement of contract contingent
  
  on an event not happening
  Contingent contracts to do or not to do anything if an uncertain
  
  future event does not happen, can be enforced when the happening of that event
  
  becomes impossible, and not before.
   Illustration 
  A agrees to pay B a sum of money if a certain ship does not
  
  return. The ship is sunk. The contract can be enforced when the ship sinks.