Halls (2.5 points)
==================

A hall for cultural events (theatre hall, conference room, etc)
represents its seating area with a nested list in which each sublist
represents a row with the corresponding seats. A seat is represented
by a string (:py:class:`str`) with values that can be ``'FREE'``,
``'OCCU'`` and ``'NAVA'``, which stand for, respectively, *free seat*,
*occupied seat* and *seat not available* (due to the applied covid
limited capacity). Rows may have different lenghts.

For different cultural events or new capacity limitations, the seat
values may change and also some rows can be extended with one seat. This
changes are represented by a dictionary (:py:class:`dict`) in which
the key is a seat position and the value is one of the three strings
described in the previous paragraph. A seat position is represented by
a :py:class:`tuple` with 2 values (:py:class:`int`), :math:`(r, s)`,
corresponding to the row and the seat, respectively, and such that
:math:`0 \leq r<ln`, :math:`ln` being the length of the nested list, and
:math:`0 \leq s \leq lr`, :math:`lr` being the length of the corresponding row; 
:math:`s=lr` means that we must add a seat with the given value at the
end of the row.

Save the following function to file ``halls.py``.

Write a **modifier** function :py:func:`halls` that takes a nested
:py:class:`list` and a dictionary (:py:class:`dict`) as
those described, and **modifies** the nested list taking into account all the
changes given in the dictionary. Examples:
   
   .. literalinclude:: test-halls.txt
      :language: python
      :lines: 3-13

   .. note:: You have more tests in file :download:`test-halls.txt`