code: def cirs_xyz(self, epoch):
          
          if isinstance(epoch, Time):
              pass
          elif isinstance(epoch, float):
              epoch = Time(None, tt=epoch)
          elif epoch == 'date':
              epoch = self.t
          else:
              raise ValueError('the epoch= must be a Time object,'
                               ' a floating point Terrestrial Time (TT),'
                               ' or the string "date" for epoch-of-date')
  
          vector = einsum('ij...,j...->i...', epoch.C, self.position.au)
          return Distance(vector)
docs: Compute cartesian CIRS coordinates at a given epoch (x, y, z).
  
          Calculate coordinates in the Celestial Intermediate Reference System
          (CIRS), a dynamical coordinate system referenced to the Celestial
          Intermediate Origin (CIO). As this is a dynamical system it must be
          calculated at a specific epoch.