[ $a => $b ]->[ $b <= $a ]

##</code><code>##

[ $a => $b ]->[ $b <= $a ]

is same as

[ $a, $b ]->[ ($b <= $a) ]

which either evaluates to

[ $a, $b ]->[0]  # returning $a
or
[ $a, $b ]->[1]  # returning $b