* https://py-tutorial-de.readthedocs.io/de/python-3.3/introduction.html#benutzung-von-python-als-taschenrechner
  * http://www.tutorialspoint.com/python/python_numbers.htm
  * http://www.python-kurs.eu/python3_operatoren.php
  * http://docs.python.org/3.4/library/stdtypes.html#numeric-types-int-float-complex