:- table avg/2.

sum(X,[H|T]) :- sum(Y,T), H \= null, Y \= null, X is H + Y.
sum(H,[H|T]) :- sum(null,T), H \= null.
sum(X,[null|T]) :- sum(X,T).
sum(null,[]).

?- sum(N, [1,2,3]).

count(X,[H|T]) :- count(Y,T), H \= null, X is Y + 1.
count(X,[null|T]) :- count(X,T).
count(0,[]).

?- count(N, [1,2,3]).

avg(X,L) :- sum(Y,L), count(C,L), Y \= null, C \= 0, X is Y / C.
avg(null,L) :- sum(Y,L), Y = null.
avg(null,L) :- count(C,L), C = 0.
avg(null,[]).

?- avg(N, [1,2,3]).

min(Y,[H|T]) :- min(Y,T), H \= null, Y \= null, H > Y.
min(H,[H|T]) :- min(Y,T), H \= null, Y \= null, H =< Y.
min(H,[H|T]) :- min(null,T), H \= null.
min(X,[null|T]) :- min(X,T).
min(null,[]).

max(Y,[H|T]) :- max(Y,T), H \= null, Y \= null, H =< Y.
max(H,[H|T]) :- max(Y,T), H \= null, Y \= null, H > Y.
max(H,[H|T]) :- max(null,T), H \= null.
max(X,[null|T]) :- max(X,T).
max(null,[]).