code: def _update_cache(self):
          
          expected_version = (
              tuple(r._version for r in self.registries) +
              (self._extra_registry._version, ))
  
          if self._last_version != expected_version:
              registry2 = Registry()
  
              for reg in self.registries:
                  registry2.key_bindings.extend(reg.key_bindings)
  
              # Copy all bindings from `self._extra_registry`.
              registry2.key_bindings.extend(self._extra_registry.key_bindings)
  
              self._registry2 = registry2
              self._last_version = expected_version
docs: If one of the original registries was changed. Update our merged
          version.