[plt-scheme] Contracts on Libraries

Hello all,

I'd like some comments on addings contracts to libraries. I'm
developing a graph library, soon on planet, which on the interface
module contains contracts on the functions the user might access.
In my opinion this is good because it allows me to separate the code
doing real stuff and the checks (does the function receive a struct
and blablabla). However, I'd like to know if people agree with the use
of contracts on libraries or if for efficiency reasons (or any other),
think that no contracts should be used and that they should only
appear on user code.

Cheers,

-- 
Paulo Jorge Matos - pocm at soton.ac.uk
http://www.personal.soton.ac.uk/pocm
PhD Student @ ECS
University of Southampton, UK