Function selected_constant_terms 

fn selected_constant_terms<S: PixSpec>(
    constant_terms: &[PixGroupRepr<S>],
    runs: impl IntoIterator<Item = (PolynomialIndex, PolynomialIndex)>,
) -> Vec<(PolynomialIndex, PixGroupRepr<S>)>

Picks out the constant-term commitments covered by runs, in ascending polynomial-index order.

Each run is an inclusive (first, last) polynomial-index pair. Runs are expected ascending and disjoint — which is what the wire decoder enforces and what the Exit’s own scope produces — so this concatenates rather than deduplicating, and a caller passing overlapping runs merely repeats entries the peer then discards as re-deliveries.

Indices past the end are skipped rather than refused: a peer naming a polynomial this cycle does not have is asking for something that cannot exist, and there is nothing to send for it.