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Z←L⍳R returns a nested array of indices suitable to indexing L.
L is an array of rank not equal 1.
R is an arbitrary array.
Z is a nested array of shape ⍴R whose items are each integer vectors of length ⍴⍴L, suitable for use as indices to L, except for where the item in R is not found in L, in which case the corresponding item in Z is ⎕IO+⍴L.
This feature extends dyadic iota to non-vector left arguments.
This function is sensitive to ⎕IO and ⎕CT.

For example, in origin-1

      M←2 3⍴'abcdef'
 1 1  2 3  3 4
      L←2 ⋄ ⎕fmt L⍳⍳3
│┌0┐ ┌0┐ ┌0┐│
││0│ │0│ │0││
│└~┘ └~┘ └~┘2

Note that this extension preserves the identity R≡L[L⍳R] for all R⊆L.

This extension is implemented via an internal magic function:

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