let $a = a, $b = b
remember a ^ a = 0, a ^ 0 = a, (a^b)^c = a^(b^c), a^b=b^a

                 $a                   $b
            ------------------ -------------------- 
$a=$a^$b        a^b                    b
$b=$a^$b        a^b           (a^b)^b=a^(b^b)=a^0=a
$a=$a^$b     a^b^a=a^a^b=0^b=b         a