Here is a more complex example of using s-tags with ``YYFILL`` to parse a file
with IPv4 addresses. Tag variables are part of the lexer state, and they are
adjusted in ``YYFILL`` like other input positions.
Note that it is necessary for s-tags because their values are invalidated after
shifting buffer contents. It may not be necessary in a custom implementation
where tag variables store offsets relative to the start of the input string
rather than buffer, which may be the case with m-tags.
