docs: def _policy_psets(policy_instances):
      """Find all permission sets making use of all of a list of policy_instances.
      The input is an array of policy instances.
  
      """
      if len(policy_instances) == 0:
          # Special case: find any permission sets that don't have
          # associated policy instances.
          return PermissionSet.objects.filter(policyinstance__isnull=True)
      else:
          return PermissionSet.objects.filter(
              policyinstance__policy__in=policy_instances).distinct()