module letting_go_and_taking_hold
  depends on unobstructing_and_obstructing

  sort declarations
    let_go_of :: unobstruct
    take_hold :: obstruct

  function declarations
    fluents 
      basic
        is_held : tangible_entity -> booleans
        held_by : tangible_entity * tangible_entity -> booleans

        abuts : spatial_entity * spatial_entity -> booleans
 
  axioms
    is_held(X) if held_by(X, Y).
    -held_by(X, Y) if is_held(X).
  
    is_obstructed(X) if is_held(X).
    -is_obstructed(X) if -is_held(X).

    abuts(X, Y) if held_by(X, Y).


    false if instance(X, let_go_of),
             object(X, O),
             -instance(X, tangible_entity).

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

    false if instance(X, let_go_of),
             agent(X, A),
             -instance(A, tangible_entity).

    false if instance(X, take_hold),
             object(X, O),
             -instance(X, tangible_entity).

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

    false if instance(X, take_hold),
             agent(X, A),
             -instance(A, tangible_entity).


    occurs(X) causes -is_held(O) if instance(X, let_go_of),
                                    object(X, O),
                                    -defined_agent(X).

    occurs(X) causes -held_by(O, A) if instance(X, let_go_of),
                                       object(X, O),
                                       agent(X, A).

    occurs(X) causes is_held(O) if instance(X, take_hold),
                                    object(X, O),
                                    -defined_agent(X).

    occurs(X) causes held_by(O, A) if instance(X, take_hold),
                                      object(X, O),
                                      agent(X, A).


    impossible occurs(X) if instance(X, let_go_of),
                            object(X, O),
                            -defined_agent(X),
                            -is_held(O). 

    impossible occurs(X) if instance(X, let_go_of),
                            object(X, O),
                            agent(X, A),
                            -held_by(O, A). 

    impossible occurs(X) if instance(X, take_hold),
                            object(X, O),
                            -defined_agent(X),
                            is_held(O). 

    impossible occurs(X) if instance(X, take_hold),
                            object(X, O),
                            agent(X, A),
                            held_by(O, A).