code: public BigInteger[][] getMatrix()
      {
          BigInteger[][] ret = new BigInteger[rows.size()][rows.get(0).size()];
  
          final int height = ret.length;
          final int width = ret[0].length;
          for (int i = 0; i < height; i++)
          {
              for (int j = 0; j < width; j++)
              {
                  ret[i][j] = rows.get(i).getColumn(j);
              }
          }
  
          return ret;
      }
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