Energy consumption (3 points)
=============================

Write the **modifier** function :py:func:`energy` that takes a list,
`m`, of energy consumptions (in kWh) that correspond to the monthly
energy consumptions of a company, and **modifies** it in such a way
that each element, except the last one, now contains the difference
between the following and the current month, i. e., :math:`m_i =
m_{i+1} - m_i,  0<=i<len(m)-1`. The last element of the list
must be 0.

Save this function to file ``p3.py``.

Examples:

.. literalinclude:: test-energy.txt
      :language: python
      :lines: 3-15

.. note::

   More tests are provided in file :download:`test-energy.txt`.