docs: def get_variables(self):
          """
          Returns list of variables of the network
  
          Examples
          --------
          >>> reader = XMLBIF.XMLBIFReader("xmlbif_test.xml")
          >>> reader.get_variables()
          ['light-on', 'bowel-problem', 'dog-out', 'hear-bark', 'family-out']
          """
          variables = [variable.find('NAME').text for variable in self.network.findall('VARIABLE')]
          return variables