Filter Students
================

Consider a list of students where each student is represented by a list with two components: a DNI without letter (:class:`int`) and a list of grades in qualitative form, thus a grade is a :class:`str` formed by a letter ``A``, ``B``, ``C`` or ``D`` possibly followed by a ``+`` or ``-``. The order in decreasing value of qualitative grades is as follows: ``A+``, ``A``, ``A-``. ``B+``, ... , ``D``, ``D-``.

For example:

    .. literalinclude:: Lsts.txt
       :language: python3

Implement the following Python function in the module :mod:`filter_sts` (file :file:`filter_sts.py`):

    ``filter_sts(Lsts, gref)``
        | *takes*
        | - ``Lsts`` a non-empty list of students as specified above
        | - ``gref`` is a qualitative grade as specified above
        | *returns*
        | a list with the students in ``Lsts`` whose grades are all equal or greater than ``gref``. The list must be increasingly ordered by the DNI.

Exemples:

    .. literalinclude:: filter_sts-test.txt
       :language: python3
       :lines: 3-

.. note::  To do this function you can, but is not mandatory, import and use the ``leq_qual(g1, g2)`` function **provided** in the module :download:`qgrades.py`. Check the function code to is it useful for.

Doctests are available at the :download:`filter_sts-test.txt` file.