%  append

% append called with two lists and a variable (++-) 
% generates an answer list consisting of the 
% two lists joined together. Notice how it preserves order.

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
% | ?
%%%% cf. the same thing in ML:
% fun append [] k = k
%   | append (x :: l) k = (x :: (append l k));
% 
%%%% but ML matching is more restrictive.  Consider Prolog list
%%%% membership:  

% member(X, []) :- fail. 
% This case is not necessary, because prolog considers
%  not matching to be failure. It is here for illustrative reasons.
member(X, [X|_]).
member(X, [_|Z]) :- member(X,Z).

% | ?- member(1,[5,4,3,2,1]).
% 
% yes
% | ?- member(1,[5,4,3,2]).
% 
% no
% | ?- 
% 
%%%% The following is NOT allowed in ML:
% fun member x [] = false
%   | member x (x :: l) = true    (* here is the problem *)
%   | member x (y :: l) = member x l;
%%%% -- instead we must write:
% fun member x [] = false
%   | member x (y :: l) = if (x=y) then true else member x l;