code: public function buildCondition(array $configuration, IMetaModel $metaModel)
      {
          // No children, then return "true".
          if (empty($configuration['children'])) {
              return new NotCondition(new BooleanCondition(false));
          }
          if (1 < $count = count($configuration['children'])) {
              throw new \InvalidArgumentException('NOT conditions may only contain one child, ' . $count . ' given.');
          }
  
          return new NotCondition($this->factory->createCondition($configuration['children'][0], $metaModel));
      }
docs: {@inheritDoc}
  
  @throws \InvalidArgumentException When there are more than one children.