docs: def _add_unknown_char(self, string):
          '''
          Adds an unknown character to the stack.
          '''
          if self.has_xvowel:
              # Ensure an xvowel gets printed if we've got an active
              # one right now.
              self._promote_solitary_xvowel()
  
          self.unknown_char = string
          self._flush_char()