protected function isChildSameAsParent(array $parent, array $children)
    {
        if (count($children) === 1) {
            $child = reset($children);
            if ($parent[Tag::ItemNumber] === $child[Tag::ItemNumber]
              && $parent[Tag::Quantity] === $child[Tag::Quantity]
              && Number::isZero($child[Tag::UnitPrice])
            ) {
                return true;
            }
        }
        return false;
    }