code: def _logdetXX(self):
          
          if not self._restricted:
              return 0.0
  
          ldet = slogdet(self._X["tX"].T @ self._X["tX"])
          if ldet[0] != 1.0:
              raise ValueError("The determinant of XᵀX should be positive.")
          return ldet[1]
docs: log(｜XᵀX｜).