code: def unregister_identity(self, category, type_):
          
          key = category, type_
          if key not in self._identities:
              raise KeyError(key)
          if len(self._identities) == 1:
              raise ValueError("cannot remove last identity")
          del self._identities[key]
          self.on_info_changed()
docs: Unregister an identity previously registered using
          :meth:`register_identity`.
  
          If no identity with the given `category` and `type_` has been
          registered before, :class:`KeyError` is raised.
  
          If the identity to remove is the last identity of the :class:`Node`,
          :class:`ValueError` is raised; a node must always have at least one
          identity.