def __call__(self, asset):
raise NotImplementedError<EOL>
Subclasses have to override this method to implement the actual
        handler function code. This method is called with asset as argument.
        Depending on the type of the handler, this method must change asset
        state (as it does in :class:`~gears.processors.Directivesprocessor`)
        or return some value (in case of asset compressors).