<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _concrete_acl(self, acl_doc):
 """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` """

        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