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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_instance_list(self, instance_docs):
 """Concretize a list of instance documents. :param list instance_docs: A list of instance documents. Should come from the API. :returns: A list of :py:class:`bases.BaseInstance`s. :rtype: list """

        if not instance_docs:
            return []

        return list(
            filter(None, [self._concrete_instance(instance_doc=doc) for doc in instance_docs])
        )