Trains:
(1⍳⍨comp)
Index
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x←?1e6⍴1e5 cmpx '100 (1 ⍳⍨ ≤) x' '(100≤x)⍳1' cmpx '9e5 (1 ⍳⍨ ≤) x' '(9e5≤x)⍳1'