Remove any ``<a:gd>`` element children of ``<a:avLst>`` and replace
        them with ones having (name, val) in *guides*.

def rewrite_guides(self, guides):
        """
        Remove any ``<a:gd>`` element children of ``<a:avLst>`` and replace
        them with ones having (name, val) in *guides*.
        """
        self._remove_avLst()
        avLst = self._add_avLst()
        for name, val in guides:
            gd = avLst._add_gd()
            gd.name = name
            gd.fmla = 'val %d' % val