[plt-scheme] texpict and slideshow confusion

Hi all (again),

Last hour was searching for an error which I just found out and it
makes no sense.
Check the code:
(require (lib "class.ss")
         (prefix tp: (lib "texpict.ss" "texpict"))
         (prefix ss: (lib "slide.ss" "slideshow")))

(tp:pict? (ss:blank 10 10)) ;==> #f

(tp:cc-superimpose (ss:blank 10 10) (ss:blank 10 10)) ;==> wierd error

The error is:
pict-width: expects args of type <struct:pict>; given instance of a
different <struct:pict>

This error is terrible: first, pict-width is the function in error but
no pict-width is on sight and one cannot delve into the internals (it
seems backtrace stays on user-code). We know, however, due to colors
that the problem is in cc-superimpose.
Even so, the error is completely wierd... Kind of like: expecting a
string, but received a string. heh

I understand that the problem is that I'm using functions required
from different modules but isn't it the same structure that is just
being re-provided by slide from texpict? So, internally what's the
problem?

-- 
Paulo Jorge Matos - pocm at sat inesc-id pt
Web: http://sat.inesc-id.pt/~pocm
Computer and Software Engineering
INESC-ID - SAT Group