( ( middle3
  =   x p
    .     @(!arg:? [?p:? [(1/2*!p+-3/2) %?x [(1/2*!p+3/2) ?)
        & !x
      |   !arg
          ( !p:<3&"is too small"
          | "has even number of digits"
          )
  )
&     123 12345 1234567 987654321 10001 -10001 -123 -100 100
      -12345 1 2 -1 -10 2002 -2002 0
  : ?L
& whl'(!L:%?e ?L&out$(middle3$!e))
&
);