%
% the Yale shooting problem -- no 'dead' fluent 
% with walking 
%

% define fluents and actions 

fluent(alive).
fluent(loaded).
fluent(walking).


action(shoot).
action(load).
action(start_walk).
action(end_walk).

% define time line 

time(0..n).

% initial state 

h(alive, 0).
h(neg(loaded), 0). 
h(walking, 0).

% action effects 

% shooting causes neg(alive) only if 
% the gun is loaded and the turkey is not walking 

h(neg(alive), T+1):-
    time(T), o(shoot, T), h(loaded, T).

h(neg(loaded), T+1):-
    time(T), o(shoot, T), h(loaded, T).

% loading causes loaded 

h(loaded, T+1):-
    time(T), o(load, T).

% start walking causes the turkey to be walking 

h(walking, T+1):-
    time(T), o(start_walk, T), h(neg(walking), T). 

% end walking causes the turkey to be not walking 

h(neg(walking), T+1):-
    time(T), o(end_walk, T), h(walking, T). 

% action occurrences 

%%% o(shoot, 0).

%%% o(load, 0). o(shoot, 1).

%o(load, 0). 
%o(shoot, 1).
1 { o(A1,T1) : action(A1) } 1 :- time(T1).

:- not h(neg(alive), n).

% static causal laws 

h(neg(walking), T):-
    time(T), h(neg(alive), T).

% what was missing is the rule that say "thing do not 
% change by themselve" 

% F was true at T and is not false at T+1 then 
% F must continue to be true at T 

h(F, T+1):- 
    time(T), T<n, fluent(F), 
    h(F, T), not h(neg(F), T+1). 

% F was false at T and is not true at T+1 then 
% F must continue to be false at T 

h(neg(F), T+1):- 
    time(T), T<n, fluent(F), 
    h(neg(F), T), not h(F, T+1). 

% hiding fluent and action 

hide fluent(F).
hide action(A).

% hiding time 

hide time(T).