A newbe wants to know what this mean..

bubble_sort(Xs, Ys):-
append(Ws, [A,B|Zs], Xs),
B < A,
append(Ws, [B,A|Zs], Vs), !,
bubble_sort(Vs, Ys).
bubble_sort(Xs, Xs).

this is a very simple bubblesort algorithm, the only thing I don't
understand is the use of ,!, . I can't find what that's for, but if
I delete it the program returns a bunch of wrong solutions after the
first correct one.

thanks
Luca

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