'Save the given playlist.
The playlist must have an ``uri`` attribute set. To create a new
playlist with an URI, use :meth:`create`.
Returns the saved playlist or :class:`None` on failure.
*MUST be implemented by subclass.*
:param playlist: the playlist to save
:type playlist: :class:`mopidy.models.Playlist`
:rtype: :class:`mopidy.models.Playlist` or :class:`None`'

def save(self, playlist):

    raise NotImplementedError
