@classmethod
def _activation_url(cls, watch):
    """Return a URL pointing to a view which :meth:`activates
        <tidings.models.Watch.activate()>` a watch.

        TODO: provide generic implementation of this before liberating.
        Generic implementation could involve a setting to the default
        ``reverse()`` path, e.g. ``'tidings.activate_watch'``.

        """
    raise NotImplementedError
