docs: def get_definition(self):
          """
          Add Definition to XMLBIF
  
          Return
          ------
          dict: dict of type {variable: definition tag}
  
          Examples
          --------
          >>> writer = XMLBIFWriter(model)
          >>> writer.get_definition()
          {'hear-bark': <Element DEFINITION at 0x7f1d48977408>,
           'family-out': <Element DEFINITION at 0x7f1d489773c8>,
           'dog-out': <Element DEFINITION at 0x7f1d48977388>,
           'bowel-problem': <Element DEFINITION at 0x7f1d48977348>,
           'light-on': <Element DEFINITION at 0x7f1d48977448>}
          """
          cpds = self.model.get_cpds()
          cpds.sort(key=lambda x: x.variable)
          definition_tag = {}
          for cpd in cpds:
              definition_tag[cpd.variable] = etree.SubElement(self.network, "DEFINITION")
              etree.SubElement(definition_tag[cpd.variable], "FOR").text = cpd.variable
              for child in sorted(cpd.variables[:0:-1]):
                  etree.SubElement(definition_tag[cpd.variable], "GIVEN").text = child
  
          return definition_tag