code: def _concrete_instance_list(self, instance_docs):
          
          if not instance_docs:
              return []
  
          return list(
              filter(None, [self._concrete_instance(instance_doc=doc) for doc in instance_docs])
          )
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