Function fold_codeword 

pub fn fold_codeword<F, EF>(codeword: &[F], beta: EF) -> Vec<EF>
where
    F: FoldAlphabet<EF>,
    EF: TowerLevel,

Fold a whole codeword, halving its length.

Each block of symbols evaluates the domain once, then advances it by one exclusive-or per packed group.

Evaluating per symbol instead is the right shape for a single query.

Over a whole codeword it would recompute that value once per output symbol.

Panics

Panics unless the codeword is empty or has a power-of-two length. That is the shape every round schedule produces.

The loops themselves cover any even length, so this is a deliberate narrowing of the contract rather than a limit of the method.