docs: def _store_cached_source_variable(self, source_id, name, value):
          """
          Stores the current known value of a source variable into the cache.
          Calls any source callbacks.
          """
          source_state = self._source_state.setdefault(source_id, {})
          name = name.lower()
          source_state[name] = value
          logger.debug("Source Cache store S[%d].%s = %s",
                       source_id, name, value)
          for callback in self._source_callbacks:
              callback(source_id, name, value)