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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(self, instance_doc):
 """Concretize an instance document. :param dict instance_doc: A document describing an instance. Should come from the API. :returns: A subclass of :py:class:`bases.BaseInstance`, or None. :rtype: :py:class:`bases.BaseInstance` """

        if not isinstance(instance_doc, dict):
            return None

        # Attempt to instantiate the appropriate class for the given instance document.
        try:
            service = instance_doc['service']
            cls = self._service_class_map[service]
            return cls(instance_document=instance_doc, instances=self)

        # If construction fails, log the exception and return None.
        except Exception as ex:
            logger.exception(ex)
            logger.error(
                'Instance construction failed. You probably need to upgrade to a more '
                'recent version of the client. Instance document which generated this '
                'warning: {}'.format(instance_doc)
            )
            return None