docs: def _render_hs_label(self, hs):
          """Return the label of the given Hilbert space as a string"""
          if isinstance(hs.__class__, Singleton):
              return self._render_str(hs.label)
          else:
              return self._tensor_sym.join(
                  [self._render_str(ls.label) for ls in hs.local_factors])