Let $S$ be a stack containing $n+1$ coefficients $a_0, a_1, \dots, a_n$, where $n \ge 1$, $a_n$ is at the top of the stack, and $a_0$ is at the bottom. The following algorithm computes $f(x) = \sum_{i=0}^{n} a_i x^i$. Identify the missing expression $\square$ in the pseudocode.

POLY(S, x) 1 while S.size >= 2 2 u = S.pop() 3 v = S.pop() 4 S.push(□) 5 return S.pop()