docs: def get_parents(self):
          """
          Returns the parents of the variables present in the network
  
          Examples
          --------
          >>> reader = XMLBIF.XMLBIFReader("xmlbif_test.xml")
          >>> reader.get_parents()
          {'bowel-problem': [],
           'dog-out': ['family-out', 'bowel-problem'],
           'family-out': [],
           'hear-bark': ['dog-out'],
           'light-on': ['family-out']}
          """
          variable_parents = {definition.find('FOR').text: [edge.text for edge in definition.findall('GIVEN')]
                              for definition in self.network.findall('DEFINITION')}
          return variable_parents