YYFILL with sentinel character
------------------------------

If EOF rule is used, ``YYFILL`` is a function-like primitive that accepts
no arguments and returns a value which is checked against zero. ``YYFILL``
invocation is triggered by condition ``YYLIMIT <= YYCURSOR`` in default API and
``YYLESSTHAN()`` in generic API. A non-zero return value means that ``YYFILL``
has failed. A successful ``YYFILL`` call must supply at least one character and
adjust input positions accordingly. Limit must always be set to one after the
last input position in buffer, and the character at the limit position must be
the sentinel symbol specified by ``re2c:eof`` configuration. The pictures below
show the relative locations of input positions in buffer before and after
``YYFILL`` call (sentinel symbol is marked with ``#``, and the second picture
shows the case when there is not enough input to fill the whole buffer).

.. code-block:: text

                   <-- shift -->
                 >-A------------B---------C-------------D#-----------E->
                 buffer       token    marker         limit,
                                                      cursor
    >-A------------B---------C-------------D------------E#->
                 buffer,  marker        cursor        limit
                 token

                   <-- shift -->
                 >-A------------B---------C-------------D#--E (EOF)
                 buffer       token    marker         limit,
                                                      cursor
    >-A------------B---------C-------------D---E#........
                 buffer,  marker       cursor limit
                 token

Here is an example of a program that reads input file ``input.txt`` in chunks of
4096 bytes and uses EOF rule.
