message: Separate joint_combiner sampling from commitment absorption
diff: @@ -133,7 +133,7 @@ where
  .for_each(|c| fq_sponge.absorb_g(&c.unshifted));
  //~ 1. If lookup is used:
  - let joint_combiner = if let Some(l) = &index.lookup_index {
  + if let Some(l) = &index.lookup_index {
  let lookup_commits = self
  .commitments
  .lookup
  @@ -148,7 +148,9 @@ where
  .ok_or(VerifyError::IncorrectRuntimeProof)?;
  fq_sponge.absorb_g(&runtime_commit.unshifted);
  }
  + }
  + let joint_combiner = if let Some(l) = &index.lookup_index {
  //~~ - If it involves queries to a multiple-column lookup table,
  //~~ then squeeze the Fq-Sponge to obtain the joint combiner challenge $j'$,
  //~~ otherwise set the joint combiner challenge $j'$ to $0$.
  @@ -165,16 +167,24 @@ where
  let joint_combiner_field = joint_combiner.to_field(endo_r);
  let joint_combiner = (joint_combiner, joint_combiner_field);
  - //~~ - absorb the commitments to the sorted polynomials.
  - for com in &lookup_commits.sorted {
  - fq_sponge.absorb_g(&com.unshifted);
  - }
  -
  Some(joint_combiner)
  } else {
  None
  };
  + if let Some(_) = &index.lookup_index {
  + let lookup_commits = self
  + .commitments
  + .lookup
  + .as_ref()
  + .ok_or(VerifyError::LookupCommitmentMissing)?;
  +
  + //~~ - absorb the commitments to the sorted polynomials.
  + for com in &lookup_commits.sorted {
  + fq_sponge.absorb_g(&com.unshifted);
  + }
  + }
  +
  //~ 1. Sample $\beta$ with the Fq-Sponge.
  let beta = fq_sponge.challenge();
  