docs: def determine_vocab(self, qualifier):
          """Determine the vocab from the qualifier."""
          vocab_value = VOCAB_INDEX.get(self.tag, None)
          if isinstance(vocab_value, dict):
              if qualifier is None:
                  qualifier = 'None'
              # Find the value based on the qualifier.
              return vocab_value.get(qualifier, None)
          elif vocab_value is not None:
              return vocab_value
          else:
              return None