Piwik\Category\Category::addSubcategory PHP Method

    public function addSubcategory(Subcategory $subcategory)
    {
        $subcategoryId = $subcategory->getId();
        if ($this->hasSubcategory($subcategoryId)) {
            throw new \Exception(sprintf('Subcategory %s already exists', $subcategoryId));
        }
        $this->subcategories[$subcategoryId] = $subcategory;
    }