Neos\Eel\FlowQuery\FlowQuery::peekOperationName PHP Method

    public function peekOperationName()
    {
        if (isset($this->operations[0])) {
            return $this->operations[0]['name'];
        } else {
            return null;
        }
    }

 /**
  * {@inheritdoc}
  *
  * @param FlowQuery $flowQuery the FlowQuery object
  * @param array $arguments the filter expression to use (in index 0)
  * @return void
  * @throws FizzleException
  */
 public function evaluate(FlowQuery $flowQuery, array $arguments)
 {
     if (count($flowQuery->getContext()) === 0) {
         return;
     }
     if (!isset($arguments[0]) || empty($arguments[0])) {
         if ($flowQuery->peekOperationName() === 'filter') {
             $filterOperation = $flowQuery->popOperation();
             if (count($filterOperation['arguments']) === 0 || empty($filterOperation['arguments'][0])) {
                 throw new FizzleException('Filter() needs arguments if it follows an empty children(): children().filter()', 1332489382);
             }
             $selectorAndFilter = $filterOperation['arguments'][0];
         } else {
             throw new FizzleException('children() needs at least a Property Name filter specified, or must be followed by filter().', 1332489399);
         }
     } else {
         $selectorAndFilter = $arguments[0];
     }
     $parsedFilter = FizzleParser::parseFilterGroup($selectorAndFilter);
     if (count($parsedFilter['Filters']) === 0) {
         throw new FizzleException('filter needs to be specified in children()', 1332489416);
     } elseif (count($parsedFilter['Filters']) === 1) {
         $filter = $parsedFilter['Filters'][0];
         if (isset($filter['PropertyNameFilter'])) {
             $this->evaluatePropertyNameFilter($flowQuery, $filter['PropertyNameFilter']);
             if (isset($filter['AttributeFilters'])) {
                 foreach ($filter['AttributeFilters'] as $attributeFilter) {
                     $flowQuery->pushOperation('filter', [$attributeFilter['text']]);
                 }
             }
         } elseif (isset($filter['AttributeFilters'])) {
             throw new FizzleException('children() must have a property name filter and cannot only have an attribute filter.', 1332489432);
         }
     } else {
         throw new FizzleException('children() only supports a single filter group right now, i.e. nothing of the form "filter1, filter2"', 1332489489);
     }
 }