friend auto operator==(const slist& left, const slist& right)
{
return left.count() == right.count() && (!left.node_ || *left.node_ == *right.node_);
}
slist(expression e, slist next) : node_(std::make_shared<node>(std::move(e), std::move(next.node_))), count_(next.count() + 1) { }