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

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])
        )