Prolog Backtracking and Cuts

cuts.pl

% tail length

len([],A,A). %base
len([_|T],A,R) :-
    A1 is A + 1,
    len(T, A1, R).

% params to send 

len(L,Res):- len(L,0, Res).


%tail sum up entries to list
sum_list([],A,A). % end case
sum_list([H|T],A,R) :-
    A1 is A + H,
    sum_list(T, A1, R).

% for humans
sum_list(L,Res):- sum_list(L,0, Res).

sum_ex3.pl

sum2(0,0). % base case
sum2(Num,Sum):-
    Num > 0,
    Num1 is Num // 10,
    sum2(Num1,Sum1),
    Sum is Num mod 10 + Sum1.