Module propagate 

Forward checking and AC-FC hybrid propagation.

On a domain wipe-out these return Some(constraint_id) naming the constraint that emptied a domain — a “blame” signal. None means propagation completed without a wipe-out. Every pruned neighbor is still recorded on trail for O(touched) undo (the kernel’s touched-VarId trail).

Type Aliases

PropResult

Outcome of a propagation step: Some(ci) on wipe-out (constraint ci is to blame), None on success.