dictionary gives a hint how to do this, but this is neat
keyD =: 1 : '(=@[) (] (u >)"_ 0 <@#) ]'
1 2 3 1 3 2 1 <@i. keyD 'abcdefg'
┌─────┬───┬───┐
│0 3 6│1 5│2 4│
└─────┴───┴───┘
u is a dyadic verb, and will be called with y u (keyed y items) for each key.
Boxes to avoid fills.
-&# keyD~ 1 2 3 1 3 2 1
4 5 5
above are toy applications, but a real one is a list of records that link to
each other, and you wish to walk through the links.
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