% Source: http://www.cs.kuleuven.ac.be/~dtai/prototypes/dppd/doubleapp.html
double_app(X,Y,Z,Res) :-
	app(X,Y,Int),
	app(Int,Z,Res).

app([],L,L).
app([H|X],Y,[H|Z]) :-
	app(X,Y,Z).