% Programmet er ikke afprøvet!

location(egg, duck_pen).
location(ducks, duck_pen).
location(fox, woods).
location(you, house).

nextto(duck_pen, yard, closed).
nextto(yard, house, open).
nextto(yard, woods, open).

connect(X, Y) :- nextto(X, Y, open).
connect(X, Y) :- nextto(Y, X, open).

goto(X) :-
  location(you, L), 
  connect(L, X), 
  retract( location(you, L) ), 
  assert( location(you, X) ), 
  write(' You are in the '), write(X), nl.
goto(X) :-
  write(' You cannot get there from here. '), nl.

open(gate) :-
  retract( nextto(duck_pen, yard, _ ) ), 
  assert( nextto(duck_pen, yard, open ) ).

myclose(gate) :-
  retract( nextto(duck_pen, yard, _) ), 
  assert( nextto(duck_pen, yard, closed) ).

chase(ducks) :-
  location(ducks, yard), 
  location(you, yard), 
  retract( location(ducks, yard) ), 
  assert( location(ducks, duck_pen) ), 
  write(' The ducks are back in their pen. '), nl.
chase(ducks) :-
  write(' No ducks here. '), nl.

take(X) :-
  location(you, L), 
  retract( location(X, L) ), 
  assert( you_have(X) ), 
  write(' You now have the '), write(X), nl.
take(X) :-
  write(' There is no '), write(X), write(' here.'), nl.

ducks :-
  location(ducks, duck_pen), 
  location(you, duck_pen), 
  connect(duck_pen, yard), 
  retract( location(ducks, duck_pen) ), 
  assert( location(ducks, yard) ), 
  write(' The ducks have run into the yard. '), nl.
ducks.

fox :-
  location(ducks, yard), 
  location(you, house), 
  write(' The fox has taken a duck. '), nl.
fox.

go :- done.
go :-
  write('>> '), 
  read(X), 
  call(X), 
  ducks, 
  fox, 
  go.

done :-
  location(you, house), 
  you_have(egg), 
  write(' Thanks for getting the egg. '), nl.