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Adverse u :: v  _ _ _  

The result of u :: v is that of u, provided that u completes without error; otherwise the result is the result of v .
 

For example:
   p=: 3 1 0 2                 A permutation vector
   x=: 'ABCD'
   p{x
DBAC

   ]i=: A. p                   Atomic index in ordered list of permutations
20

   i A. x                      Permutation by atomic representation
DBAC

   q=: 3 1 1 0                 Not a permutation
   q{x
DBBA
          
   A. q
|index error
|       A.q

   A=: A. :: (!@#)             Give index outside range in case of error
   A p
20
        
   A q
24

   24 A. x
|index error
|   24   A.x


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