code: def _sign_block(self, block):
          
          block_header = block.block_header
          header_bytes = block_header.SerializeToString()
          signature = self._identity_signer.sign(header_bytes)
          block.set_signature(signature)
          return block
docs: The block should be complete and the final
          signature from the publishing validator (this validator) needs to
          be added.