law: Transfer of Property Act, 1882
  27. Conditional transfer to one person coupled with
  
  transfer to another on failure of prior disposition
  Where, on a transfer of property, an interest therein is created
  
  in favor of one person, and by the same transaction an ulterior disposition of
  
  the same interest is made in favor of another, if the prior disposition under
  
  the transfer shall fail, the ulterior disposition shall take effect upon the
  
  failure of the prior disposition, although the failure may not have occurred in
  
  the manner contemplated by the transferor.
  But, where the intention of the parties to the transaction is
  
  that the ulterior disposition shall take effect only in the event of the prior
  
  disposition failing in a particular manner, the ulterior disposition shall not
  
  take effect unless the prior disposition fails in that manner.
  Illustrations
  (a) A transfers Rs. 500 to B on condition that he shall execute
  
  a certain lease within three months after A's death, and, if he should
  
  neglect to do so, to C. B dies in A's life-time. The disposition in favor
  
  of C takes effect.
  (b) A transfers property to his wife; but, in case she should
  
  die in his life-time, transfer to B that which he had transferred to her. A and
  
  his wife perish together, under circumstances which make it impossible to prove
  
  that she died before him. The disposition in favor of B does not take effect.