code: def _check_label(self, label):
          
  
          if not isinstance(label, str):
              label = str(label)
  
          if label.lower() in _forbidden_labels:
              raise ValueError("'{}' is forbidden to be used as a label"
                               .format(label))
          if not re.match("^[a-z,A-Z,0-9,_]*$", label):
              raise ValueError("label '{}' is forbidden - only alphabetic, numeric, and '_' characters are allowed in labels".format(label))
          if len(self.filter(twig=label, check_visible=False)):
              raise ValueError("label '{}' is already in use".format(label))
          if label[0] in ['_']:
              raise ValueError("first character of label is a forbidden character")
docs: Check to see if the label is allowed.