docs: public boolean add(int e)
      {
          if(e < 0 || e >= has.length)
              throw new IllegalArgumentException("Input must be in range [0, " + has.length + ") not " + e);
          else if(contains(e) )
              return false;
          else
          {
              if (nnz == 0)
              {
                  first = e;
                  next[e] = prev[e] = STOP;
              }
              else
              {
                  prev[first] = e;
                  next[e] = first;
                  prev[e] = STOP;
                  first = e;
              }
              nnz++;
              return has[e] = true;
          }
      }