Potential issue with exercise J10

[rikedyp]

both provided solutions (In1, In2) for exercise J10 exhibit unexpected behavior due to a wrap-around issue:

            'xx' In1 'x 123 xx 321 x'
┌→───┐
│7 14│
└~───┘
            'xx' In2 'x 123 xx 321 x'
┌→───┐
│7 14│
└~───┘

[rikedyp]

To avoid the shifting issue, can use drops. These spellings use the rank operator and where function, which might not be appropriate.

R←⍸∧⌿(¯1+⍳≢X)(↓⍤0 1)X∘.=Y               ⍝ In1
R←⍸X∧.=(-¯1+⍳≢X)(↓⍤0 1)((⍴X),1+⍴Y)⍴Y   ⍝ In2

[rodrigogiraoserrao]

At that point in the book the operator rank has just been introduced, so I guess it's legitimate to use it.
To keep in line with the dfns presented, In1 could use drop: In ← {⍸∧⌿(¯1+⍳≢⍺)⌽0 ¯1↓⍺∘.=⍵}.

Not sure if you're working on a bigger rework of the problems/solutions, but this 2D drop seems to fix the two solutions without having to modify things too much.