module major_body_movement
  depends on changing_position

  sort declarations
    sit_down :: orient
    stand_up :: orient

  function declarations
    fluents
      basic
        is_sitting : entity -> booleans
        is_standing : entity -> booleans


  axioms
    false if instance(X, sit_down),
             -defined_agent(X).

    object(X, Y) if instance(X, sit_down),
                    agent(X, Y).

    occurs(X) causes is_sitting(A) if instance(X, sit_down),
                                      agent(X, A).

    occurs(X) causes -is_standing(A) if instance(X, sit_down),
                                        agent(X, A).
 
    impossible occurs(X) if instance(X, sit_down),
                            agent(X, A),
                            is_sitting(A).


    false if instance(X, stand_up),
             -defined_agent(X).

    object(X, Y) if instance(X, stand_up),
                    agent(X, Y).

    occurs(X) causes is_standing(A) if instance(X, stand_up),
                                       agent(X, A).

    occurs(X) causes -is_sitting(A) if instance(X, stand_up),
                                       agent(X, A).
 
    impossible occurs(X) if instance(X, stand_up),
                            agent(X, A),
                            is_standing(A).