% simple_travel.lp
% Gregory Gelfond <grisha.gelfond@gmail.com>

#const max = 3.
step(1..max).
max_step(3).

#domain step(Step).

% objects

vehicle(train).

#domain vehicle(Vehicle).

location(station).
position(en_route).

position(X) :-
    location(X).

#domain position(Position; Position1; Position2).

% fluents

fluent(at(Vehicle,Position)).

#domain fluent(Fluent).

% dynamic causal laws

h(at(Vehicle,en_route), Step + 1) :-
    Step < max,
    o(depart(Vehicle), Step).

% state constraints

-h(at(Vehicle,Position1), Step) :-
    h(at(Vehicle,Position2), Step),
    neq(Position1, Position2).

% axioms of intent

o(Action, Step) :-
    intended(Action, Step),
    not -o(Action, Step).

intended(Action, Step + 1) :-
    Step < max,
    intended(Action, Step),
    -o(Action, Step).

% inertia axioms

h(Fluent, Step + 1) :-
    Step < max,
    h(Fluent, Step),
    not -h(Fluent, Step + 1).

-h(Fluent, Step + 1) :-
    Step < max,
    -h(Fluent, Step),
    not h(Fluent, Step + 1).