:: Adverse

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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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