Vector – Flash CS4 Professional ActionScript 3.0 Language Reference
compareFunctionは
function compare(x:T, y:T):Number {}
で
a negative number, if x should appear before y in the sorted sequence
0, if x equals y
a positive number, if x should appear after y in the sorted sequence
と書いてあるのだけど、実はint(±１以上？）じゃないと判定してくれない？