A.21 library(intercept): Intercept and signal interface
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Availability::- use_module(library(intercept)).(can be autoloaded)
Sourceintercept(:Goal, ?Ball, :Handler, +Arg)
Similar to intercept/3, but the copy of Handler is called as call(Copy,Arg), which allows passing large context arguments or arguments subject to unification or destructive assignment. For example: 
?- intercept(send_signal(x), X, Y=X).
true.

?- intercept(send_signal(x), X, =(X), Y).
Y = x.