%\item \self SAT solvers in general use an \textit{exclusive or} between two formulas (i.e. $\phi \; \oplus \; \psi$) to check if the given formulas are \textit{equivalent}. Why does the formula $\chi = \phi \; \oplus \; \psi$ have to be "UNSAT" in order to prove that the two formulas $\phi$ and $\psi$ are \textit{equivalent}? What is the big advantage of using this method for checking \textit{equivalency} between two formulas?