Function pack_words_concrete 

pub fn pack_words_concrete<F, E>(words: &[Word]) -> Vec<E>
where
    F: BinaryField,
    E: FieldOps<Scalar = F> + From<F>,

WordIPVerifierChannel::pack_words over concrete words, for channels carrying Word.

A set bit contributes the basis element at its position in the packed layout, which is what makes this agree with reading the element’s bits back out.