Solution
= Solution
Use the <bounded formula in set theory>[bounded formula]
$$
\boxed{\forall z\in x\ (z\in y).}
$$
It expresses $x\subseteq y$ with no unbounded <quantifier>. For parameters $x,y\in U_1$, transitivity puts every member of $x$ in $U_1$ already, and membership in $y$ has the same meaning in both classes. Thus the formula is $\Delta_0$ and is absolute in both directions, in particular upward absolute.