fd-neq adds a closure to the constraint store and runs it once on
post. After every label binding, fd-fire-store re-runs all stored
constraints — when one side of a fd-neq later becomes ground, the
domain of the other side has the value removed.
Propagator semantics:
(number, number) -> equal? fail : ok
(number, var) -> remove number from var's domain
(var, number) -> symmetric
(var, var) -> defer (re-fires after each label step)
Pigeonhole-fails test confirms the constraint flow ends correctly:
3 vars all-pairwise-distinct over a 2-element domain has no solutions.
7 new tests, 593/593 cumulative.
giles
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