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Dyadic iota (L⍳R) is extended to allow 1<⍴⍴L, returning a nested array of shape ⍴R whose items are each integer vectors of length ⍴⍴R, suitable for use as indices to L.

For example, in origin-1

      M←2 3⍴'abcdef'
      M⍳'af'
 1 1  2 3
      M[M⍳'af']
af

This extension is implemented via an internal magic function:

    ∇ Z←L F R;⎕IO;O
[1]   O←⎕IO ⋄ ⎕IO←0
[2]   Z←⊂[0] O+(1+⍴L)⊤(¯1↓,(1+⍴L)↑L)⍳R
    ∇