Common characters
=================

Write the function :py:func:`common` that takes two strings ``s1`` and
``s2``, and returns another string with those characters that are both
in ``s1`` and ``s2``. In the returned string, characters must appear
in the same order than in the first given string ``s1`` and there must
not be repeated characters. Save this function into the file
``common.py``. Examples:
   
   .. literalinclude:: common.txt
      :language: python3
      :lines: 3-
             
   .. note::

      More tests are provided in file :download:`common.txt <common.txt>`

.. rubric:: Solution
      
A solution is provided in file :download:`common.py <solutions/common.py>`