'Generate (char, style) tuples backward from start to stop
a la range(start, stop, -1) -- i.e. exclusive at \'stop\' index.
For SciMozAccessor this can be implemented more efficiently than
the naive usage of char_at_pos()/style_at_pos().'

def gen_char_and_style_back(self, start, stop):

    raise VirtualMethodError()
