Can someone explain what the prolog does with this recursive program?

Question

I recently started to study prolog, but it's hard for me to deal with recursive rules. I understand the simple rules, but I am having problems with this example, which I found in a program that gives the sum of all the elements in a list:

addup([], 0).

addup([FirstNumber | RestOfList], Total) :-
    addup(RestOfList, TotalOfRest),   
    Total is FirstNumber + TotalOfRest.

Now, if I trace this, I get the following:

[trace]  ?- addup([3, 5, 7], Total).
   Call: (7) addup([3, 5, 7], _G322)
   Call: (8) addup([5, 7], _L1)
   Call: (9) addup([7], _L2)
   Call: (10) addup([], _L3)
   Exit: (10) addup([], 0)    % I understand what it does till here
^  Call: (10) _L2 is 7+0
^  Exit: (10) 7 is 7+0
   Exit: (9) addup([7], 7)
^  Call: (9) _L1 is 5+7
^  Exit: (9) 12 is 5+7
   Exit: (8) addup([5, 7], 12)
^  Call: (8) _G322 is 3+12
^  Exit: (8) 15 is 3+12
   Exit: (7) addup([3, 5, 7], 15)
Total = 15.

; TotalOfRest , , . TotalOfRest ( _L3) 0. ? , L2, 7 + 0. , , , ? 7? , , TotalOfRest? RestofList [] FirstNumber 7?

, : L3 ?

, , - , !

Answer 1

, :

addup([], 0).
addup([FirstNumber | RestOfList], Total) :-
    addup(RestOfList, TotalOfRest),   
    Total is FirstNumber + TotalOfRest.

TotalOfRest. , TotalRest , .

trace , :

[trace]  ?- addup([3, 5, 7], Total).
   Call: (7) addup([3, 5, 7], _Total)
   Call: (8) addup([5, 7], _TotalOfRest1)
   Call: (9) addup([7], _TotalOfRest2)
   Call: (10) addup([], _TotalOfRest3)
   Exit: (10) addup([], 0)
^  Call: (10) _TotalOfRest2 is 7+0
^  Exit: (10) 7 is 7+0
   Exit: (9) addup([7], 7)
^  Call: (9) _TotalOfRest1 is 5+7
^  Exit: (9) 12 is 5+7
   Exit: (8) addup([5, 7], 12)
^  Call: (8) _Total is 3+12
^  Exit: (8) 15 is 3+12
   Exit: (7) addup([3, 5, 7], 15)
Total = 15.

, , , _TotalOfRest1. _TotalOfRest3. _TotalOfRest3 = 0 0. , : Total is FirstNumber + TotalOfRest.. , Total ( TotalOfRest FirstNumber). . , a Total , , _TotalOfRest2 .

, :

addup([3, 5, 7], Total) :-
    % FirstNumber = 3
    % RestOfList = [5, 7]
    % Total = Total
    addup([5,7], _TotalOfRest1) :-
         % FirstNumber = 5
         % RestOfList = [7]
         % Total = _TotalOfRest1
         addup([7], _TotalOfRest2) :-
             % FirstNumber = 7
             % RestOfList = []
             % Total = _TotalOfRest2
             addup([],_TotalOfRest3),
             % resolved to _TotalOfRest3 = 0
             Total is 7 + 0.
             % resolved to Total = 7
         % resolved to _TotalOfRest2 = 7
         Total is 5 + 7.
         % resolved to Total = 12
     % resolved to _TotalOfRest1 = 12
     Total is 3 + 12.
     % resolved to Total = 15
% resolved tot Total = 15

, , , .