code: public function addCallback(VariationInterface $variation)
      {
          $closure = (function () {
              if (!$this->isValid()) {
                  $this->doRewind();
              }
  
              return $this->getCurrent();
          })->bindTo($variation, $variation);
          $this->contextCallbacks[$variation->getName()] = $closure;
  
          return true;
      }
docs: @param VariationInterface $variation
  
  @return bool