weird XPCE threading issue ?

Some time ago I answered this question (
http://stackoverflow.com/questions/12158293/xpce-for-prolog-in-compiled-files/12165153#12165153),
and wrote this snippet:

:- use_module(library(pce)).
:- use_module(library(plot/barchart)).
:- use_module(library(autowin)).

test_barchart :-
    new(W, picture),
    send(W, display, new(BC, bar_chart(vertical,0,200))),
    forall(member(Name/Height/Color,
              [x/100/red, y/150/green, z/80/blue, v/50/yellow]),
           (   new(B, bar(Name, Height)),
               send(B, colour(Color)),
               send(BC, append, B)
           )),
    send(W, open).

Now I tried again that code, and I was left to stare at a blinking cursor
on an empty line...

After some test I found that commenting out, in ~/.plrc

:- set_prolog_flag(xpce_threaded, true).

it worked again. I feel the problem it's over my head...

Carlo
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