(
(fun swap (p)
   (local (t (fst p))
      (block
         (setfst p (snd p))
         (setsnd p t)
         p)))
)
(local (x (pair 1 2))
    (local (y x)      
       (block                 { boxed semantics:       unboxed semantics:  }
          (write x)           {     (1.2)                  (1,2)           }
          (write y)           {     (1.2)                  (1,2)           }
          (setfst y 3)        
          (write x)           {     (3,2)                  (1,2)           }
          (write y)           {     (3,2)                  (3,2)           }
          (@swap x)        
          (write x)           {     (2,3)                  (1,2)           }
          (write y))))        {     (2,3)                  (3,2)           }