avg_([], Sum, Len, Avg) :- 
	Avg is Sum / Len.
avg_(Arr, Sum, Len, Avg) :-
	append([Car], Cdr, Arr),
	Sum2 is Sum + Car,
	Len2 is Len + 1,
	avg_(Cdr, Sum2, Len2, Avg).
avg(Arr, Avg) :-
	avg_(Arr, 0, 0, Avg).

main :- 
    Lst1 = [1,2,3,4,5,6,7,8,9],
    avg(Lst1, Avg),
    print(Avg),
    halt.

:- main.