code: private static int aggregateStamp(int stamp_a, int stamp_b) {
          if (stamp_a == UNSET || stamp_b == UNSET) {
              return UNSET;
          }
          return (stamp_a > stamp_b) ? stamp_a : stamp_b;
      }
docs: Returns the pseudo-time-stamp for two fields, given their
  individual pseudo-time-stamps.  If either of the fields
  is unset, then the aggregate is unset.  Otherwise, the
  aggregate is the later of the two stamps.