Jan Burse, created Sep 15. 2018
* The system predicate sys_unbind/1 installs an unbind handler and
* immediately succeeds. The unbind handler is invoked during a redo or
* a close. In contrast to a cutter the unbind handler is not invoked
* when choice points are removed. When the unbind handlers are executed
* exceptions are aggregate_alld that are possibly thrown by the unbind
* handlers. When unbind handler fails a directive failed error is thrown.
* The predicate sys_freeze_var/2 will create a new reference object that
* captures the given variable. The reference object will have a stable
* hash, equal and lexical ordering for the duration of the continuation.
* The predicate sys_melt_var/2 allows retrieving the term the variable
* is currently instantiated to. The predicate sys_bound_var/1 allows
* checking whether the variable is currently instantiated or not.
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* The predicate installs an unbind handler A and immediately succeeds.
* The unbind handler is invoked during a redo or a close.
* sys_freeze_var(V, R):
* The predicate succeeds when R unifies with a new references
* object that captures the variable V.
% sys_freeze_var(+Var, -Ref)
sys_freeze_var/2, 'SpecialUndo', 1).
* sys_melt_var(R, T):
* The predicate succeeds when T unifies with the variable captured
* by the reference object R.
% sys_melt_var(+Ref, -Var)
sys_melt_var/2, 'SpecialUndo', 2).
* The predicate succeeds when the variable captured by the reference
* object R is currently instantiated.
sys_bound_var/1, 'SpecialUndo', 3).
* The predicate succeeds when R is a reference to a variable.