code: static RollupStats getOrNull(Vec vec, final Key rskey ) {
      Value val = DKV.get(rskey);
      if( val == null )           // No rollup stats present?
        return vec.length() > 0 ? /*not computed*/null : /*empty vec*/new RollupStats(0);
      RollupStats rs = val.get(RollupStats.class);
      return rs.isReady() ? rs : null;
    }
docs: Fetch if present, but do not compute