Consider the following fixed length MAC for messages of length $\ell(n)=2n-2$ using a pseudorandom function $F$:
On input of a mesage $m_0||m_1$ ($|m_0| = |m_1| = n-1$) and a key $k \in \{0,1\}^n$, algorithm Mac outputs $t=F_k(0||m_0)||F_k(1,m_1)$. Algorithm Vrfy is defined in the natural way.
Is (Gen, Mac, Vrfy) existentially unforgeable under a chosen message attack? Why?