if_replaced - dæmon predicate invoked when a slot value is replaced
SlotName:
if_replaced predicate
The if_replaced dæmon is triggered when the replace predicate is used to alter the value of a slot. The dæmon is ordinary Prolog code. The special function old value can be used to access the value of the slot prior to replacement and new value can be used to access the value being used to replace the old.
replace, new value