'How does the predicate i/4 function in Prolog

In "Representation and Inference for Natural Language - A First Course in Computational Semantics" by Blackburn and Bos they introduce the predicate i/4:

i(X,model(_,F),G,Value):-
    (
       var(X),
       memberList(g(Y,Value),G),
       Y==X, ! % IMPORTANT CUT!
    ;
      atom(X),
      memberList(f(0,X,Value),F)
    ).

They briefly mentioned that the predicate i/4 interprets terms using the variable assignment g, if the term is a variable, and with the interpretation function F, if it is a constant, but they do not address the "IMPORTANT CUT!" comment.

I don't understand what the funtion G represents and I also don't fully understand the importance of the line Y==X, !

I tried to run the code without the line Y==X, !,to figure out what it does, but it just gives me an error.

I also know that Y==X returns either 0 (when X != Y) or 1 (when X = Y),but I don't don't know what the , ! part is supposed to mean.

nlp prolog

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'How does the predicate i/4 function in Prolog

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