code: public function prime($key, $value)
      {
          $this->checkKey($key, __METHOD__);
  
          $cacheKey = $this->getCacheKeyFromKey($key);
  
          // Only add the key if it does not already exist.
          if (!$this->promiseCache->has($cacheKey)) {
              // Cache a rejected promise if the value is an Error, in order to match
              // the behavior of load(key).
              $promise = $value instanceof \Exception ? $this->getPromiseAdapter()->createRejected($value) : $this->getPromiseAdapter()->createFulfilled($value);
  
              $this->promiseCache->set($cacheKey, $promise);
          }
  
          return $this;
      }
docs: {@inheritdoc}