Function set_columns 

pub fn set_columns(on: bool) -> bool

Turn the columnar layout on or off for this process, and answer what it was.

docs/08 §8.3 item 8’s off switch. Nothing observable changes: with it off every list is Seq::Boxed, every answer is the same answer, and the difference is memory. That is exactly why it is worth having — the switched-off path is what a gate compares against, so “the two layouts are one list” is measured rather than asserted.