Upper Selected Letters
======================

You are required to deliver the following function in the module :mod:`upper_module` (file :file:`upper_module.py`):

    .. py:function:: upper_sel(text, ref)

such that
  
    **given**
        - *text*, :class:`str`
        - *ref*, :class:`str` of lowercase letters

    **returns** a :class:`str` like *text* where
        - all *letters* occuring in *ref* are in upper case.
        - all characters that are not alphanumeric are replaced by a space (``' '``).

For example:

    .. literalinclude:: upper_sel.test
        :language: python3
        :start-after: --ini
        :end-before: --fi

.. note:: Observe that digits remain untouched.

Doctests are available in the :download:`upper_sel.test` file.