src/FACTFinder/Adapter/Recommendation.php
public function addProductIDs($productIDs)
    {
        $parameters = $this->request->getParameters();
        $parameters->add('id', $productIDs);
        $this->upToDate = false;
    }
public function addProductIDs($productIDs)
    {
        $parameters = $this->request->getParameters();
        $parameters->add('id', $productIDs);
        $this->upToDate = false;
    }
Add one or multiple product IDs to base recommendation on, in addition to
any IDs previously set.

@param string|string[] $productIDs One or more product IDs.