docs: def get_variables(self):
          """
          Add variables to XMLBIF
  
          Return
          ------
          dict: dict of type {variable: variable tags}
  
          Examples
          --------
          >>> writer = XMLBIFWriter(model)
          >>> writer.get_variables()
          {'bowel-problem': <Element VARIABLE at 0x7fe28607dd88>,
           'family-out': <Element VARIABLE at 0x7fe28607de08>,
           'hear-bark': <Element VARIABLE at 0x7fe28607de48>,
           'dog-out': <Element VARIABLE at 0x7fe28607ddc8>,
           'light-on': <Element VARIABLE at 0x7fe28607de88>}
          """
          variables = self.model.nodes()
          variable_tag = {}
          for var in sorted(variables):
              variable_tag[var] = etree.SubElement(self.network, "VARIABLE", attrib={'TYPE': 'nature'})
              etree.SubElement(variable_tag[var], "NAME").text = var
          return variable_tag