[mlochbaum]

In the BQN compiler I'd write (⊂¯1++⍀m)⌷(m←c=LF)⌿ as the equivalent of (⊂⌈⍀m×⍳≢m←c=LF)⌷ using the function IT (indices-times but it also includes the ⌈⍀). Although I'm not sure it's inherently faster. Most of the cost is in selection, and there's an algorithm taking a shuffle of a sliding window from the right argument that would work better with the denser indices. And those indices have a smaller type which may be useful. The annoying thing is that this whole set-and-hold operation could be implemented in SIMD directly and none of those complications would apply. But it's pretty hard to expose in a nice way without just providing a builtin.