code: def get_checked(self):
          
          checked = []
  
          def get_checked_children(item):
              if not self.tag_has("unchecked", item):
                  ch = self.get_children(item)
                  if not ch and self.tag_has("checked", item):
                      checked.append(item)
                  else:
                      for c in ch:
                          get_checked_children(c)
  
          ch = self.get_children("")
          for c in ch:
              get_checked_children(c)
          return checked
docs: Return the list of checked items that do not have any child.