mpyw\Co\Internal\GeneratorContainer::key PHP Method

    public function key()
    {
        $this->validateValidity();
        return $this->g->key();
    }

 /**
  * Handle resolving generators still running.
  * @param  GeneratorContainer $gc
  * @return PromiseInterface
  */
 private function processGeneratorContainerRunning(GeneratorContainer $gc)
 {
     // Check delay request yields
     if ($gc->key() === CoInterface::DELAY) {
         return $this->pool->addDelay($gc->current())->then(function () use($gc) {
             $gc->send(null);
             return $this->processGeneratorContainer($gc);
         });
     }
     // Now we normalize yielded value
     $yielded = YieldableUtils::normalize($gc->current());
     $yieldables = YieldableUtils::getYieldables($yielded, [], $this->runners);
     if (!$yieldables) {
         // If there are no yieldables, send yielded value back into generator
         $gc->send($yielded);
         // Continue
         return $this->processGeneratorContainer($gc);
     }
     // Chain resolver
     return $this->promiseAll($yieldables, $gc->key() !== CoInterface::SAFE)->then(YieldableUtils::getApplier($yielded, $yieldables, [$gc, 'send']), [$gc, 'throw_'])->then(function () use($gc, $yieldables) {
         // Continue
         $this->runners = array_diff_key($this->runners, $yieldables);
         return $this->processGeneratorContainer($gc);
     });
 }