docs: def get_states(self):
          """
          Returns the states of variables present in the network
  
          Examples
          --------
          >>> reader = XMLBIF.XMLBIFReader("xmlbif_test.xml")
          >>> reader.get_states()
          {'bowel-problem': ['true', 'false'],
           'dog-out': ['true', 'false'],
           'family-out': ['true', 'false'],
           'hear-bark': ['true', 'false'],
           'light-on': ['true', 'false']}
          """
          variable_states = {variable.find('NAME').text: [outcome.text for outcome in variable.findall('OUTCOME')]
                             for variable in self.network.findall('VARIABLE')}
          return variable_states