Letter counting
===============

Save the following functions into a file named ``letters.py``.

#. Write the function :py:func:`occurrences` that takes a text
   (:class:`str`), and returns a dictionary (:class:`dict`) where the
   key is a lowercase letter or a digit and the value is the number of
   occurrences of it in the given text. Letters in the given text may
   be upper or lowercase. This dictionary will not contain any letter
   that does not appear in the given string, i. e., we must build an
   incomplete dictionary.
   
   See the following examples:
                 
   .. literalinclude:: test-occurrences.txt
       :language: python
       :lines: 3-11
   
   .. note::
      More tests are provided in file :download:`test-occurrences.txt`.

#. Write the function :py:func:`vowel_occurrences` that takes a text
   (:class:`str`), and returns a dictionary (:class:`dict`) where the
   key is a lowercase vowel and the value is the number of occurrences
   of it in the given text. Letters in the given text may be upper
   or lowercase. This dictionary will contain all five vowels
   regardless of wether they are in the text or not, i. e., the
   dictionary must be complete.
   
   See the following examples:
                 
   .. literalinclude:: test-vowel_occurrences.txt
       :language: python
       :lines: 3-9
   
.. note::
   More tests are provided in file :download:`test-vowel_occurrences.txt`.

.. rubric:: Solution
      
A solution of these functions is provided in file :download:`letters.py <solutions/letters.py>`