* https://py-tutorial-de.readthedocs.io/de/python-3.3/controlflow.html#for-anweisungen
  * http://www.python-kurs.eu/python3_for-schleife.php
  * https://docs.python.org/3.4/tutorial/controlflow.html#for-statements
  * https://docs.python.org/3.4/tutorial/datastructures.html#looping-techniques