An example showing that forward checking is different than arc-concistency look ahead. This is what full (and in this case also partial) look ahead does: variables are considered in pairs, so that x3 and x4 cannot be possibly be both assigned a consistent value because of the constraint between them.
No machine-readable source provided. Own work assumed (based on copyright claims).
No machine-readable author provided. Tizio assumed (based on copyright claims).