Question on propagation

Hi,

   I think I'm about to ask a very basic question,
   but I couldn't figure out what's going on in the
   following code.


[eclipse 3]: [X,Y]::[1..10], #<=(X,Y,B), X #<= Y.

B = _449{[0, 1]}
X = _423{[1..10]}
Y = _436{[1..10]}

Delayed goals:
        #>=(_436{[1..10]} - _423{[1..10]}, 0, _449{[0, 1]})
        _436{[1..10]} - _423{[1..10]}#>=0
yes.


   I thought that B would be set to 1. Why not?
   Is there a way to make this happen without doing search/labeling?

   Thank you for any clarifying comments.

Best regards,
Tallys