Indexing
From NARS2000


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.  
f is an arbitrary function.  
 
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 origin1
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


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. Negative indexing is available only when the first element of ⎕FEATURE is set to 1. 
For example, in origin1
⎕FEATURE[1]←1 V←'123'(⊂4 5) V[¯1 (0 ⍬ ¯1)] 123 4