* http://www.python-kurs.eu/python3_variablen.php
  * https://en.wikibooks.org/wiki/Python_Programming/Variables_and_Strings
  * http://www.tutorialspoint.com/python/python_basic_syntax.htm
  * https://docs.python.org/3/reference/lexical_analysis.html#identifiers
  * https://docs.python.org/2/reference/lexical_analysis.html#identifiers