code: public function parseLiteral($valueNode, ?array $variables = null)
      {
          if ($valueNode instanceof EnumValueNode) {
              $lookup = $this->getNameLookup();
              if (isset($lookup[$valueNode->value])) {
                  $enumValue = $lookup[$valueNode->value];
                  if ($enumValue !== null) {
                      return $enumValue->value;
                  }
              }
          }
  
          // Intentionally without message, as all information already in wrapped Exception
          throw new Exception();
      }
docs: @param Node         $valueNode
  @param mixed[]|null $variables
  
  @return null
  
  @throws Exception