• 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