code: public function getMatrixByRow($i0 = null, $iF = null)
      {
          if (is_int($i0)) {
              if (is_int($iF)) {
                  return $this->getMatrix($i0, 0, $iF + 1, $this->n);
              }
  
              return $this->getMatrix($i0, 0, $i0 + 1, $this->n);
          }
  
          throw new CalculationException(self::ARGUMENT_TYPE_EXCEPTION);
      }
docs: getMatrixByRow.
  
  Get a submatrix by row index/range
  
  @param int $i0 Initial row index
  @param int $iF Final row index
  
  @return Matrix Submatrix