整數s i:  0 _ _ 索引 Of Last

i: y is list of 整數s from -y to y . Thus:
   i: 3
_3 _2 _1 0 1 2 3

   i: _2
2 1 0 _1 _2

   i: 0
0
  i: is like i. but gives 索引 of last occurrence.
 

例如:
   1 2 3 4 1 2 3 i: 1 2 2 3 3 3 4 4 5
4 5 5 6 6 6 3 3 7

   1 2 3 4 1 2 3 i. 1 2 2 3 3 3 4 4 5
0 1 1 2 2 2 3 3 7

   (i.~ ,: i:~) 'boustrophedonic'
0  1 2 3 4 5  1 7 8 9 10  1 12 13 14
0 11 2 3 4 5 11 7 8 9 10 11 12 13 14

   (3 # i.3 4) i: (i.2 4)
2 5

   (3 # i.3 4) i. (i.2 4)
0 3

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