* https://py-tutorial-de.readthedocs.io/de/python-3.3/datastructures.html#mehr-zu-bedingungen
  * https://docs.python.org/3.4/library/stdtypes.html?highlight=boolean#truth-value-testing
  * http://www.mediasonics.ch/_archiv/programming_python/projekt/06.html
  * http://www.tutorialspoint.com/python/python_basic_operators.htm