🚀 go-pugleaf

Did something go wrong in the Eureka step here? What does the variable
X do?
	double(X0, X, Y0, Y) :-
		double(X0, X, Y0, Y1, Y1, Y).

	double(X, X, Y, Y, Z, Z).
	double([E|X1], X, [E|Y1], Y, [E|Z1], Z) :-
		double(X1, X, Y1, Y, Z1, Z).

Why not write ;

        double(X0,Y0,Y):-
                double(X0,Y0,Y1,Y1,Y).

        double([],Y,Y,Z,Z).
        double([E|X1],[E|Y1],Y,[E|Z1],Z):-
               double(X1,Y1,Y,Z1,Z).