code: public void add(int left, IntBitSet right) {
          if (line[left] == null) {
              line[left] = new IntBitSet();
          }
          line[left].addAll(right);
      }
docs: Adds a set of pairs.
  @param  left   left value of all pairs to add
  @param  right  right values of the pairs to add. If empty, no
  pair is added