Normally, once a constraint has been posted, it remains active until its entailment has been detected, or it has been backtracked over. In special circumstances, it may be desirable to completely forget any constraints attached to a given variable, as if they never had been posted. The following predicate can be used for that purpose:
fd_purge(+Var) since release 4.6
Given a domain variable Var, any constraints and domain information attached to Var are erased. Var ceases to be a domain variable.