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Having list append implementation in Prolog -

append([],X,X). append([H|T1],X,[H|T2]) :- append(T1,X,T2). 

Which gives on -

append([a,b,c],[d,e,f],X). 

The output -

X = [a,b,c,d,e,f]. 

I Wonder how does it work , I tried to trace function calling but I didn't understand how T2 , which is the tail of a list , can be has as tail of X which in append([a,b,c],[d,e,f],X) calling had not defined yet . Can you clear up this recursion for me ?

Having list append implementation in Prolog -

append([],X,X). append([H|T1],X,[H|T2]) :- append(T1,X,T2). 

Which gives on -

append([a,b,c],[d,e,f],X). 

The output -

X = [a,b,c,d,e,f]. 

I wonder how does it work , I tried to trace function calling but I didn't understand how T2 , which is the tail of a list , can be has as tail of X which in append([a,b,c],[d,e,f],X) calling had not defined yet . Can you clear up this recursion for me ?