Swi Prolog keeps track of recursion depth

Why does recursion depth start at 6 or 7 when tracing Prolog programs?

Example (taken from the form here ).

factorial(0, 1).
factorial(N, NFact) :-
    N > 0,
    Nminus1 is N - 1,
    factorial(Nminus1, Nminus1Fact),
    NFact is Nminus1Fact * N.

      

        
        
        
      

    

Allows you to trace factorial(3, X)

. Output:

Call: (7) factorial(3, _G284) ? creep*
Call: (8) 3>0 ? creep
Exit: (8) 3>0 ? creep
Call: (8) _L205 is 3-1 ? creep 

      

        
        
        
      

    

etc. The first column ( Call, Exist

) is the ports, the third is the targets, and the second is the recursion depth. But why does it start at 7? (When I run the example on my machine, it starts at 6) Shouldn't it start at 1 or 0?