docs: public Set<Long> forWhomSiteIsDead(long hsid) {
          ImmutableSet.Builder<Long> isb = ImmutableSet.builder();
          Set<Long> deadBy = m_dead.get(hsid);
          if (   !deadBy.isEmpty()
              && m_survivors.contains(hsid)
              && m_strategy == ArbitrationStrategy.MATCHING_CARDINALITY) {
              isb.addAll(Sets.filter(deadBy, amongSurvivors));
          }
          return isb.build();
      }