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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _concrete_acl_list(self, acl_docs):
 """Concretize a list of ACL documents. :param list acl_docs: A list of ACL documents. Should come from the API. :returns: A list of :py:class:`ACL` objects. :rtype: list """

        if not acl_docs:
            return []

        return list(filter(None, [self._concrete_acl(acl_doc=doc) for doc in acl_docs]))