Recassim Succession
===================

Let's consider an infinite mathematical succession defined for a given :math:`a_0 \geq 0` by the following recursive rules:

.. math::
   
  a_i=
    \begin{cases}
    a_0, &\mbox{if } i = 0
    \\
    a_{i-1} - i, &\mbox{if } t = (a_{i-1}-i) \mbox{ is such that } t>0 \mbox{ and } t \mbox{ does no belong to the succession so far.}
    \\
    a_{i-1} + i, &\mbox{otherwise}
    \end{cases}


for all :math:`i \geq 0`. For instance, given :math:`a_0 = 0` the initial terms of the succession are:
$$ 0,1,3,6,2,7,13,20,12,21,11,⋯ $$

Observe that, on the one hand the 3rd term is 6 since the 2nd term is 3 and 3-3 is not `>0` therefore :math:`t_3 = 3+3 = 6`.
On the other hand, :math:`t_4 = 2` since :math:`t_3−4 = 2`.

The next image graphically shows the first 100 terms for :math:`a_0 = 0`:

.. image:: RSeqPlot.png
   :scale: 100%
   :align: center


Please implement the following Python function in the :mod:`recassim` module (:file:`recassim.py` file):

   .. py:function:: : recassim(a0, n):
	      
such that

   **given**

      - ``a0``, an :class:`int` such that ``a0 >= 0``.
      - ``n``, an :class:`int` such that belongs to the succession.

   **returns** the index of the first occurrence of ``n`` in the succession (notice that a value may appear more than once).

For example,
   
.. literalinclude:: recassim.test
      :language: python
      :start-after: --ini
      :end-before: --fi
		    

Doctests are available at the :download:`recassim.test <recassim.test>` file.