code: protected boolean combine(Object[] srcs, TupleWritable dst) {
      assert srcs.length == dst.size();
      for (int i = 0; i < srcs.length; ++i) {
        if (!dst.has(i)) {
          return false;
        }
      }
      return true;
    }
docs: Return true iff the tuple is full (all data sources contain this key).