Predicates to delay the execution of the (exit) port of a goal. This is useful to introduce side-effects at selected points (e.g., clean-ups before goal exit in failure or exception conditions).
Exit port of a goal execution, defined as:
Execute once(Goal) (alternatives are cut) and obtain its execution port Port, which can be continued with port_call/1.
The sequence (once_port_reify(Goal,P),Cleanup,port_call(P)) is semantically equivalent to once(Goal), but executes Cleanup goals in case of success, failure, and exception.