46.3.3 Deinitializing the Prolog Engine From VB

The Visual Basic interface should be deinitialized: you should call PrologDeInit() before exiting e.g. from a Form_Unload method.

This procedure was introduced in SICStus 3.10.1. Earlier versions never deinitialized the SICStus engine. This could cause problems, e.g. by leaving memory allocated and by not giving foreign resources a chance to clean up through their deinit functions.