Indexing

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Z←R[L], R[L]←A, and R[L]f←A are all extended to allow both Reach and Scatter indexing.
L is a nested array of integer scalars and/or vectors, suitable for indexing R.
R is an arbitrary array.
A is an arbitrary array.
Reach Indexing:   If L⊃R is valid, it is equivalent to ⊃R[⊂L], and
if L⊃¨⊂R is valid, it is equivalent to R[L]
Scatter Indexing:   If L⌷R is valid, it is equivalent to R[⊃∘.,/L], and
if L⌷¨⊂R is valid, it is equivalent to ⊂¨R[⊂¨L]
Both Reach and Scatter indexing may appear together within a single instance of R[L], R[L]←A, and R[L]f←A
These functions are sensitive to ⎕IO.


For example, in origin-1

      V←'123'(⊂4 5)
      V[1 (2 ⍬ 1)]
 123 4

      M←2 2⍴(10 20) (30 40 'abc') 50 60
      M[(1 1)((1 2) 3)]
 10 20  abc


Z←R[L], R[L]←A, R[L]f←A, L⌷R, L⍉R, and L⊃R are all extended to allow negative values in L.
For all but transpose, L is a nested array of integer scalars and/or vectors, suitable for indexing R; for transpose, L is an integer scalar or vector of integers, suitable for transposing R.
That is, if the largest allowed value for L is N, then the previous allowable range of values was ⎕IO to N, inclusive. Now, the allowable range of values is 1 ¯1[1]-N to N, inclusive. For example, A, A[⍳⍴A], A[⍳-⍴A], and even A[⍳¯1 1[?(⍴⍴A)⍴2]×⍴A] are all identical for any array A in either origin.
Also, A, (⍳⍴⍴A)⍉A, and (⍳-⍴⍴A)⍉A are all identical for any array A in either origin.
R is an arbitrary array.
A is an arbitrary array.
These functions are sensitive to ⎕IO.


For example, in origin-1

      V←'123'(⊂4 5)
      V[¯1 (0 ⍬ ¯1)]
 123 4