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']}

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