What does construction s() stand for in Prolog?

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artificial-intelligence

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I have a piece of example code in Prolog. I wonder what the predicate/rule/functor s() means in Prolog? (on line that starts with breadth_search). I did not see any declaration of this construction above example code. It might be a system predicate. Who knows?

main(Decision) :-
     initial_state(State),
     breadth_search([s(State,[])],[],[],Decision),
     writeln(Decision).

What I have tried:

Tried to find declaration in example code of this predicate/rule s(), but I did not find any reference to it. It might be a predefined predicate