docs: def get_properties(self):
          """
          Add property to variables in XMLBIF
  
          Return
          ------
          dict: dict of type {variable: property tag}
  
          Examples
          --------
          >>> writer = XMLBIFWriter(model)
          >>> writer.get_property()
          {'light-on': <Element PROPERTY at 0x7f7a2ffac1c8>,
           'family-out': <Element PROPERTY at 0x7f7a2ffac148>,
           'hear-bark': <Element PROPERTY at 0x7f7a2ffac188>,
           'bowel-problem': <Element PROPERTY at 0x7f7a2ffac0c8>,
           'dog-out': <Element PROPERTY at 0x7f7a2ffac108>}
          """
          variables = self.model.nodes()
          property_tag = {}
          for var in sorted(variables):
              properties = self.model.node[var]
              property_tag[var] = etree.SubElement(self.variables[var], "PROPERTY")
              for prop, val in properties.items():
                  property_tag[var].text = str(prop) + " = " + str(val)
          return property_tag