Module freeze
:- use_module(library(freeze)). Provides the constraint freeze/2.
freeze(Var, Goal)
Schedules Goal to be executed when Var is instantiated. This can be useful to observe the exact moment a variable becomes bound to a more concrete term, for example when creating animations of search processes. Higher-level constructs such as phrase_from_file/2 can also be implemented with freeze/2, by scheduling a goal that reads additional data from a file as soon as it is needed.