Prolog looping due to interdependency

[keowang]

Have to write some prolog rules to represent the geometrical properties about isosceles triangle.

First rule is to say

angleEqual(angle(A,B,C),angle(A,C,B)):- 
    triangle(A,B,C), 
    lineEqual(line(A,B),line(A,C)).

Second rule says

lineEqual(line(A,B),line(A,C)):- 
    triangle(A,B,C), 
    angleEqual(angle(A,B,C),angle(A,C,B)).

But the two rules together will cause a interdependency and prolog is looping forever if given facts such as

triangle(a,b,c).
lineEqual(line(a,b),line(a,c)).

and query about

?- angleEqual(A,B).

Any idea? Thanks very much.