\item \self Construct a ROBDD for the formula

$$f = (p \leftrightarrow q) \land (r \leftrightarrow s)$$

using \textit{variable order} $r < s < p < q$. Use complemented edges and a node for \texttt{true} as the only constant node. To simplify drawing, you may assume that \textit{dangling edges} point to the constant node. Write down all cofactors that you compute to obtain the final result and mark them in the graph.
