def __call__(request, subpath, kw):
    """
        Modifies a subpath and/or keyword arguments from which a static asset
        URL will be computed during URL generation.

        The ``subpath`` argument is a path of ``/``-delimited segments that
        represent the portion of the asset URL which is used to find the asset.
        The ``kw`` argument is a dict of keywords that are to be passed
        eventually to :meth:`~pyramid.request.Request.static_url` for URL
        generation.  The return value should be a two-tuple of
        ``(subpath, kw)`` where ``subpath`` is the relative URL from where the
        file is served and ``kw`` is the same input argument. The return value
        should be modified to include the cache bust token in the generated
        URL.

        The ``kw`` dictionary contains extra arguments passed to
        :meth:`~pyramid.request.Request.static_url` as well as some extra
        items that may be usful including:

          - ``pathspec`` is the path specification for the resource
            to be cache busted.

          - ``rawspec`` is the original location of the file, ignoring
            any calls to :meth:`pyramid.config.Configurator.override_asset`.

        The ``pathspec`` and ``rawspec`` values are only different in cases
        where an asset has been mounted into a virtual location using
        :meth:`pyramid.config.Configurator.override_asset`. For example, with
        a call to ``request.static_url('myapp:static/foo.png'), the
        ``pathspec`` is ``myapp:static/foo.png`` whereas the ``rawspec`` may
        be ``themepkg:bar.png``, assuming a call to
        ``config.override_asset('myapp:static/foo.png', 'themepkg:bar.png')``.
        """
