Map *slide* to an integer representing its zero-based position in
        this slide collection. Raises |ValueError| on *slide* not present.

def index(self, slide):
        """
        Map *slide* to an integer representing its zero-based position in
        this slide collection. Raises |ValueError| on *slide* not present.
        """
        for idx, this_slide in enumerate(self):
            if this_slide == slide:
                return idx
        raise ValueError('%s is not in slide collection' % slide)