append([], List, List) :- !.     % cut as confirming the choice of rule.
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
% | ?- append(X, Y, [the,cat,sat,on,the,mat]).
% 
% X = []
% Y = [the,cat,sat,on,the,mat] ;
% 
% no
% | ?- 


% The ! (cut operator) prevents backtracking from 
% reconsidering alternative options for this predicate after the
% cut has been passed. A cut commits the search
% to the predicate choice and the variable assignments made so far.

% The pressing of the ; key now results in failure,
% since there are no alternative choices to be made. The example
% here is of a red cut, there are two types of cut, red and green. 

% A green cut does not alter the search but makes it more efficient by reducing
% unnecessary search. Green cuts are typically used to prevent backtracking from
% trying mutually exclusive cases which are known to be impossible.

% Red cuts fundamentally alter the program search.
% A red cut prevents a search from reaching certain 
% areas of the search, and alters the behaviour of the predicate. 
% Cuts should be carefully considered before being used.