code: def represent_as(self, new_pos, new_vel=None):
          
  
          if self.ndim != 3:
              raise ValueError("Can only change representation for "
                               "ndim=3 instances.")
  
          # get the name of the desired representation
          if isinstance(new_pos, str):
              pos_name = new_pos
          else:
              pos_name = new_pos.get_name()
  
          if isinstance(new_vel, str):
              vel_name = new_vel
          elif new_vel is None:
              vel_name = pos_name
          else:
              vel_name = new_vel.get_name()
  
          Representation = coord.representation.REPRESENTATION_CLASSES[pos_name]
          Differential = coord.representation.DIFFERENTIAL_CLASSES[vel_name]
  
          new_pos = self.pos.represent_as(Representation)
          new_vel = self.vel.represent_as(Differential, self.pos)
  
          return self.__class__(pos=new_pos,
                                vel=new_vel,
                                frame=self.frame)
docs: Represent the position and velocity of the orbit in an alternate
          coordinate system. Supports any of the Astropy coordinates
          representation classes.
  
          Parameters
          ----------
          new_pos : :class:`~astropy.coordinates.BaseRepresentation`
              The type of representation to generate. Must be a class (not an
              instance), or the string name of the representation class.
          new_vel : :class:`~astropy.coordinates.BaseDifferential` (optional)
              Class in which any velocities should be represented. Must be a class
              (not an instance), or the string name of the differential class. If
              None, uses the default differential for the new position class.
  
          Returns
          -------
          new_psp : `gala.dynamics.PhaseSpacePosition`