code: def _concrete_acl(self, acl_doc):
          
          if not isinstance(acl_doc, dict):
              return None
  
          # Attempt to instantiate an Acl object with the given dict.
          try:
              return Acl(document=acl_doc, acls=self)
  
          # If construction fails, log the exception and return None.
          except Exception as ex:
              logger.exception(ex)
              logger.error('Could not instantiate ACL document. You probably need to upgrade to a '
                           'recent version of the client. Document which caused this error: {}'
                           .format(acl_doc))
              return None
docs: Concretize an ACL document.
  
          :param dict acl_doc: A document describing an ACL entry. Should come from the API.
          :returns: An :py:class:`Acl`, or None.
          :rtype: :py:class:`bases.BaseInstance`