code: def get_dfdx(self):
          
          if self._dfdx is True:
              if self.indep is None:
                  zero = 0*self.be.Dummy()**0
                  self._dfdx = self.be.Matrix(1, self.ny, [zero]*self.ny)
              else:
                  self._dfdx = self.be.Matrix(1, self.ny, [expr.diff(self.indep) for expr in self.exprs])
          elif self._dfdx is False:
              return False
          return self._dfdx
docs: Calculates 2nd derivatives of ``self.exprs``