Is there a SELECT WHERE... IN equivalent in Xquery?

I am new to Xquery and having a hard time dealing with this problem. Basically, my current XML file's structure is like this:

<root>
   <a>
      <aid>XX</aid>
      ...
   </a>
   <a>
      <aid>xx</aid>
      ...
   </a>
   <b>
      <bid>xx</bid>
      ...
   </b>
   <b>
      <bid>xx</bid>
      ...
   </b>
...
</root>

What I am trying to get is the information of the b elements whose "bid" do not equal to any "aid" from the a elements. I tried to do it like this. First I let a variable to hold the aid:

let $aid := (xpath to the aid elements)

then I tried to do something like the "SELECT WHERE IN" from traditional sql. namely:

for $b in (xpath to b elements)

where $b/bid != $aid (I tried to achieve the NOT IN effect)

return $b/(infomation)

But the problem is I can't find the right method to do this. I also tried

where $e/bid != $aid[*]

, it didn't work either. So am I on a wrong track? Is there anyway to achieve this in Xquery(1.0)? Or is there other methods to get the required infomation?