code: public function visit(Criterion $criterion, CriterionVisitor $subVisitor = null)
      {
          if (!isset($criterion->criteria[0]) ||
               (count($criterion->criteria) > 1)) {
              throw new \RuntimeException('Invalid aggregation in LogicalNot criterion.');
          }
  
          return '(*:* NOT ' . $subVisitor->visit($criterion->criteria[0]) . ')';
      }
docs: Map field value to a proper Solr representation.
  
  @param Criterion $criterion
  @param CriterionVisitor $subVisitor
  
  @return string