'Set representation and/or differential class for this frame\'s data.
Parameters
base : str, `~astropy.coordinates.BaseRepresentation` subclass, optional
The name or subclass to use to represent the coordinate data.
s : `~astropy.coordinates.BaseDifferential` subclass, optional
The differential subclass to use to represent any velocities,
such as proper motion and radial velocity.  If equal to \'base\',
which is the default, it will be inferred from the representation.
If `None`, the representation will drop any differentials.'

def set_representation_cls(self, base=None, s=u'base'):

    if (base is None):
        base = self._representation[u'base']
    self._representation = _get_repr_classes(base=base, s=s)
