% A further example of using append to concatenate 
% two lists. 

append([], List, List).
append([Head|Tail], List, [Head|Rest]) :-
     append(Tail, List, Rest).

% | ?- append([the,cat],[sat,on,the,mat], R).
% 
% R = [the,cat,sat,on,the,mat] 
% 
% yes


% In the following example the predicate is called
% in the --+ mode (two variables and an input list). 
% The  ; key is used several times to 
% force a failure, this causes backtracking to 
% search for a different solution. Each re-done 
% solution is related to the previous solution by 
% reconsidering the last choice made. 

% To make the explanation clearer, 
% I will refer to the two cases of the append predicate
% as the base case (the first) and the recursive case (the second)
% The first result is simply the match against the first 
% (base) case. The second result is where backtracking causes a 
% reconsideration of the last choice. The alternative choice
% is to use the second case. Thus the second result 
% is after one recursion using the
% recursive case and finishing with the base case, 
% the third result is after two recursions finishing 
% with the base case, etc etc.


% | ?- append(X, Y, [the, cat, sat, on, the, mat]).
% 
% X = []
% Y = [the,cat,sat,on,the,mat] ;
% 
% X = [the]
% Y = [cat,sat,on,the,mat] ;
% 
% X = [the,cat]
% Y = [sat,on,the,mat] ;
% 
% X = [the,cat,sat]
% Y = [on,the,mat] 
% 
% yes
% | ?-