code: def _get_head_block(self, request):
          
          if request.head_id:
              if self._id_regex.fullmatch(request.head_id) is None:
                  LOGGER.debug('Invalid head id requested: %s', request.head_id)
                  raise _ResponseFailed(self._status.NO_ROOT)
              try:
                  return self._block_store[request.head_id]
              except KeyError as e:
                  LOGGER.debug('Unable to find block "%s" in store', e)
                  raise _ResponseFailed(self._status.NO_ROOT)
  
          else:
              return self._get_chain_head()
docs: Fetches the request specified head block, or the chain head.
  
          Note:
              This method will fail if `_block_store` has not been set
  
          Args:
              request (object): The parsed protobuf request object
  
          Returns:
              Block: the block object at the head of the requested chain
  
          Raises:
              ResponseFailed: Failed to retrieve a head block