code: public E determinant(E[][] matrix, int r, int s, int i, int j)
      {
          E a, b, c, d;
          a = matrix[r][s];
          b = matrix[r][j];
          c = matrix[i][s];
          d = matrix[i][j];
          E ad  = multiply(a, d); // matrix[r][s], matrix[i][j]);
          E bc = multiply(b, c); // matrix[i][s], matrix[r][j]);
          E ret = subtract(ad, bc);
  
          return ret;
      }
docs: Return the determinant.
  @param matrix array
  @param r index
  @param s index
  @param i index
  @param j index
  @return element determinant