Ex5 13

$\def\abs#1{|#1|}\def\i{\mathbf {i}}\def\ket#1{|{#1}\rangle}\def\bra#1{\langle{#1}|}\def\braket#1#2{\langle{#1}|{#2}\rangle}\def\tr{\mathord{\mbox{tr}}}\mathbf{Exercise\ 5.13}$

Show that the swap circuit of section 5.2.4 does indeed swap two single-qubit values in that it sends $\ket\psi\ket\phi$ to $\ket\phi\ket\psi$ for all single-qubit states $\ket\psi$ and $\ket\phi$.