successor arithmetics - What are the steps that Prolog follows with the definition of natural numbers? -

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I am learning how to program in prologue and I got the next program that defines natural numbers and their amount: / p> 

  yoga (succ (x), y, succ (jade)): - zodiac (x, y, z). Amount (0, X, X)? - Yoga (succ (succ (0)), succ (succ (succ (0)), answer). Answer = succ (succ (succ (succ (succ (succ (succ (0)))   
 (found)  
 The problem is that I struggled with execution I do not know what this does for the flow of this program. How can Prolog find the value of the answer?  
 
   
 
 It helps to understand how Prolog works to detect a current masterpiece, or when someone makes a new design when you make a query like :    sum (succ (succ (0)), succ (succ (succ (0)), answer).   
 Prolog facts and rules  sum (_ , _, _)  ( sum / 3 ) and select first a match in which the rules are in place:  
  (1) Yoga (succ (x), y, succ (z)): - zodiac (x, y, z). (2) zodiac (0, x, x).   
 If you look at the query, then it clearly matches the pattern of rule # 1. Remember that in Prologue, a variable can be started for any type of term, and A Are integrated variable names (immediately) for the same price. When Prong unites the "head" of # 1 with the query, it does by uniting the variable as follows:  
  x = succ (0) y = succ (succ (succ ( 0 (A) Answer = Suk (Z)   
 Note that the  answer  value  succ (Z)  even if  Z  has not been compelled (a solid value has been assigned) So far, we now follow the rule, which will query  yoga (x, y, z)  which That would be the query:  
  sum (succ (0), succ (succ (succ (0)), z) | one SZZ   
 The prolol will now start at the top because it is a new query for  sum / 3  like the first time, it rules with the following triggers Matches # 1:  
  X = 0 Y = succ (succ (succ (0))) (B) Z = succ (Z ')   
 I'm using it at   Z  to differentiate it from  sum (succ (0)), Succ ( above) succ (succ (0)), z , because it is different from the one used in the head for  sum (..., su) (su) (z)) < / code>. It looks like there is a function in C, which is declared as  int f (x) {return 2 * x; }  and you call it from somewhere other local variables  x , that name is used in two different places and two different - represents different variables).  
 Then we can follow the next recursive query,  sum (x, y, z ')  again, that becomes:  
  Sum (0, succ (succ (succ (0)), z ') | | | XYZ '  
 This recurring query does not match the rule # 1 because the first argument,  0 ,  does not match succ (X) . However, this rule matches the # 2:  
  0 = 0x = succ (succ (succ (0))) (c) x = z '  
 now  X = succ (succ (succ (0)))  hence  Z '= succ (succ (succ (0))) . Since there is no other question within this rule, it is finally successful from the place where the inquiry was done. Return it back to (B):  
  Z = succ (Z ') = succ (succ (succ (succ (0))))   < p> and return it to (a) back:  
  answer = succ (Z) = succ (succ (succ (succ (0)))))   
 And there you have it. Using @ Code> trace, you can see these steps in Prol Interpreter and immediately follow the variable.

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