Consider two sequences $u_n$ and $v_n$ defined mutually by $u_0 = 1$, $v_0 = 0$, and for $n \geq 1$: $u_n = u_{n-1} - v_{n-1}$ $v_n = u_{n-1} + v_{n-1}$ Find the value of $u_8 + v_8$.