basic question about jQuery.support.positionFixed

basic question about jQuery.support.positionFixed

Hi
At http://gist.github.com/362170 I found Paul Irish's jQuery.support.positionFixed plugin, which tells you  whether a browser supports `position:fixed`.
 
For me at least, it only works if I include it in the body element; including it in the head element results in calls to the function always coming back as undefined.
 
However, instructions on using plugins in general often state that they should be included in the head element. 
 
Am I missing something obvious or is this plugin somehow a special case? -in that, it works only when included in the body.
 
Thanks in advance for any thoughts