law: Indian Contract Act, 1872 
  34. When event on which contract is contingent
  
  to be deemed impossible, if it is the future conduct of a living person
  If the future event on which a contract is contingent is the way
  
  in which a person will act at an unspecified time, the event shall be
  
  considered to become impossible when such person does anything which renders it
  
  impossible that he should so act within any definite time, or otherwise than
  
  under further contingencies.
   Illustrations 
  A agrees to pay B a sum of money if B marries C. C marries D.
  
  The marriage of B to C must now be considered impossible, although it is
  
  possible that D may die and that C may afterwards marry B.