law: Indian Contract Act, 1872 
  58. Alterative promise, one branch
  
  being illegal
  In the case of an alternative promise, one branch of which is
  
  legal and the other illegal, the legal branch alone can be enforced.
   Illustration 
  A and B agree that A shall pay B 1,000 rupees, for which B shall
  
  afterwards deliver to A either rice or smuggled opium.
  This is a valid contract to deliver rice, and a void agreement
  
  as to the opium.