Labeling outside of fd_labeling?

Dear All,

Small question. I just posed the following query:

?- X #< Y, Y #< Z, Z #< X.

The interpreter then was busy for around ~4 secs
and then responded with "no".

The "no" is actually correct, by transitivity we have
X #< Z which conflicts with Z #< X.

What I wonder is what keeps the interpreter busy for
~4 secs and whether the "no" is reliable.

Bye