%% WARNING -- DANGER !!

answer (42).  %% this is an error!!

%% Prolog does not allow the space in the above rule!




%% Example family code (Note - you need to paste this into
%%    the simpsons.pl file, so that the rules are present!


sibs( X, Y ) :- parent(A,X), parent(A,Y), X\==Y.  

aunt( A, N ) :- parent( Q, N ), sibs( Q, A ), female( A ).

notaunt( N, A ) :- person(N), person(A),  \+ aunt( N, A ).


notoldest( X ) :- age(X,AX), age(Y,AY), AY > AX.
oldest( X ) :- person(X), \+notoldest(X).


rel( X, Y ) :- anc( X, Y ) ; anc( Y, X ).
rel( X, Y ) :- anc( Z, X ), anc( Z, Y ).




%% Example list code

%
% member( E, L ) should be true if E is a top-level member of the list L
% member should work when E is a variable or bound; L should be bound
%
member( E, [E|_] ).
member( E, [_|R] ) :- member( E, R ).


%
% reverse( L, Rev ) should be true if Rev is the reverse of the list L
% reverse should work when Rev is a variable or bound; L should be bound
%
reverse( [], [] ).   % base case ~ they don't always involve [] in Prolog!
reverse( [F|R], Rev ) :- reverse( R, Tmp ), append( Tmp, [F], Rev ).


% a helper predicate to define a graph
graph1(  [ [a,b], [b,c], [c,d], [b,w], [w,x], [x,z], [b,z] ] ).

% a helper predicate to define a tree
tree1( [ 42, [ 20, [10, [5, [], []], [18, [], []]], [] ],
             [ 100, [67, [50, [], []], []], [150, [], [2009, [], []]] ] ] ).

tree2( [ 42, [20, [], []], [] ] ).


fac( 0, 1 ).
fac( X, N ) :- Xnew is X-1, fac( Xnew, Nnew ), N is X * Nnew.

len( [], 0 ).
len( [_|R], N ) :- len( R, LenR ), N is LenR + 1.

app( [], M, M ).
app( [F|R], M, [F|App] ) :- app( R, M, App ).

nnodes( [], 0 ).
nnodes( [Root, L, R], N ) :- nnodes( L, NL ), nnodes( R, NR ), N is 1 + NL + NR.


lastof( E, [E] ).
lastof( E, [_|R] ) :- lastof( E, R ).

rev( [], [] ).
rev( [F|R], Rev ) :- rev( R, RevR ), append( RevR, [F], Rev ).

treefind( E, [E, _, _] ).
treefind( E, [F, L, R] ) :- treefind( E, L ) ; treefind( E, R ).

kid( Par, Graph, K ) :- member( [Par,K], Graph ).  % Succinct!


%% showing off var and nonvar

reverseFun( L, Rev ) :- var(L), var(Rev),
    write( 'If you insist...' ), nl,
    L = [42], Rev = [42].

reverseFun( L, Rev ) :- var(L), nonvar(Rev),
                        reverseFun(Rev, L).

reverseFun( [], [] ).

reverseFun( [F|R], Rev ) :- nonvar(F), nonvar(R),
                            reverseFun( R, RRev ), append( RRev, [F], Rev ).