code: public boolean add(int left, int right) {
          if (line[left] == null) {
              line[left] = new IntBitSet();
          } else {
              if (line[left].contains(right)) {
                  return false;
              }
          }
  
          line[left].add(right);
          return true;
      }
docs: Adds a new pair to the relation.
  @param  left   left value of the pair
  @param  right  right value of the pair
  @return false, if the pair was already element of the relation