Indices
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R is a simple array of nonnegative integers.  
For vector R, Z is an integer vector. For all other ranks of R, Z is a nested vector of integer vectors. In both case the length of Z is +/,R.  
For scalars or vectors, the result is equivalent to R/⍳⍴R which encapsulates a very common idiom in one symbol.  
For higher rank arrays, the result extends to produce a nested vector of vectors of the indices of all the positive integer elements of R replicated as per the corresponding value in R.  
This function is sensitive to ⎕IO. 
For example, in origin0
⍸,3 0 0 0 ⍬⍬⍬≡⍸3 ⍝ for scalar S, ⍸S ←→ S⍴⊂⍬ as per the definition R/⍳⍴R 1 ⍬≡⍸⍬ 1 ⍸1 0 1 1 1 0 1 0 2 3 4 6 ⍸'Now is the time'=' ' 3 6 10 ⍸2 3 4 0 0 1 1 1 2 2 2 2 ⍸⎕←2 3⍴⍳4 0 1 2 3 0 1 0 1 0 2 0 2 1 0 1 0 1 0 1 2 ⍸1 2 3⍴⍳4 0 0 1 0 0 2 0 0 2 0 1 0 0 1 0 0 1 0 0 1 2 ∇ Z←(Txt Rep) txtrep Z;a [1] ⍝ Replace Txt in Z with Rep. [2] :Assert 2=⍴⍴Z ⋄ :Assert (⍴Txt)≡⍴Rep [3] a←⍸Txt⍷Z [4] Z[⊃⊃¨,¨/¨a+⊂0(0..¯1+⍴Txt)]←((⍴a),⍴Rep)⍴Rep ∇ 'Now' 'Who' txtrep 4 13⍴'Now is the time...' Who is the ti me...Who is t he time...Who is the time.