[newbie question] How to compare boolean expression/results?

[Carlos Nepomuceno]

Hi guys! This is my first post to this list.

I'm not really learning Prolog at the moment but I started to compare how 
different programming languages treat the following floating point comparison 
case:


a=1
b=1/(1-epsilon)
(a==b) == (b==a)

That's basic a test to check if the equality operator ('==') is commutative.

I found it really hard to implement that simple test in Prolog. I've read a lot 
of stuff and the only thing I've found to work (AFAIK) was the following:

%works in GNU Prolog 1.4.0 
(http://www.compileonline.com/execute_prolog_online.php)
:- initialization(main).
equal(X,Y,Z) :- (X=:=Y -> Z=true; Z=false).
commutative(X,Y,Z) :- ((equal(X,Y,W), equal(Y,X,W)) -> Z=true; Z=false).
main :- A = 1,
        B = 1/(1-2.220446049250313e-16),
        commutative(A,B,T),
        write(T).
%output: true

That seems to be very verbose and inefficient solution.

How can I do it simpler? Is it possible to make it faster?