@ — AtA@I Y replaces selected items with A, returning a new array.
0@2 4⊢⍳5 ⍝ 1 0 3 0 5
f@I Y applies f to the selection, then puts it back.
⌽@2 4⊢⍳5 ⍝ 1 4 3 2 5
A function right operand computes a Boolean selection mask.
0@(2∘|)⍳5 ⍝ 0 2 0 4 0
Indices include nested paths. Dyadic calls pass the left argument to the replacement function. Repeated indices are written in order; the last replacement wins.