system.include() not working?

Hi all,

I tried to use system.include() to include common js functions but it doesn't seem to work. Neither using relative/absolute paths to the fs nor urls is working. I tried:

system.include("prototype.js"); => prototype.js sits in .resource
system.include("C:\path\to\servicename.resource\prototype.js");
system.include("http://ajax.googleapis.com/ajax/libs/prototype/1.6.0.3/prototype.js");

Nothing worked.
What's the right way to include common functions?

Greetings, Jens