How to solve a problem with an infinite loop [beginners question]

Hi everybody,

I have the following grammar

gleich(a,b).
gleich(b,c).

gleich(A,B) :-
A=B.

gleich(A,B) :-
gleich(B,A).

gleich(A,B) :-
gleich(A,X),gleich(X,B).

I want to ask prolog the following questions (which should all lead to
the answer true):

gleich(a,a).
gleich(a,b).
gleich(b,a).
gleich(a,c).
gleich(c,a).

gleich(a,c) and gleich(c,a) leads to an infinity loop how can I avoid
this?

best wishes

Philipp

.

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