For $i, k \in N a t$, define
$$
r(k, i)=[\text { if } k=0 \text { then } 0 \text { else } r(k-1, i)+i] .
$$
Prove that, for any $i, k \in N a t, r(i, k)=k i$. The proof should be based on the material in Section 3.2. [Caution: Be sure to keep track of the uses of $i$ and $k$ in the definition and in the assertion to be proved.]