fn apply_columns<'a>(
columns: &[usize],
vector: impl Fn(usize) -> &'a [Complex],
apply: &impl Fn(&[Complex], usize) -> Result<Vec<Complex>>,
) -> Result<Vec<Complex>>Expand description
Pack active columns only when there is more than one: scalar solves and the tail of a batch lend their existing Krylov or answer buffer directly.