'The class used for part of this frame\'s data.
Parameters
which : (\'base\', \'s\', `None`)
The class of which part to return.  \'base\' means the class used to
represent the coordinates; \'s\' the first derivative to time, i.e.,
the class representing the proper motion and/or radial velocity.
If `None`, return a dict with both.
Returns
representation : `~astropy.coordinates.BaseRepresentation` or `~astropy.coordinates.BaseDifferential`.'

def get_representation_cls(self, which=u'base'):

    if (not hasattr(self, u'_representation')):
        self._representation = {u'base': self.default_representation, u's': self.default_differential}
    return (self._representation[which] if (which is not None) else self._representation)
