Bug in reified symbolic FD

I expected the following code to bind X to the value 'a':

[eclipse 2]: B isd X #= a, B = 1.

X = X
B = 1

Delayed goals:
        #=(X, a, 1)
yes.

It knows the constraint `X #= a' is true, but fails to bind X unless X
has had a domain assigned (before B is bound):

[eclipse 3]: X :: [a, b], B isd X #= a, B = 1.

X = a
B = 1
yes.
[eclipse 4]: B isd X #= a, X :: [a, b], B = 1.

X = a
B = 1
yes.
[eclipse 5]: B isd X #= a, B = 1, X :: [a, b].

B = 1
X = X{[a, b]}

Delayed goals:
        #=(X{[a, b]}, a, 1)
yes.

Warwick