module exposing_and_concealing
  depends on accessibility

  sort declarations
    expose :: make_accessible 
    conceal :: make_inaccessible

  function declarations
    fluents 
      basic
        is_concealed : tangible_entity -> booleans
        concealed_from : tangible_entity * thing -> booleans

  axioms
    -is_accessible(X) if is_concealed(X).
    is_accessible(X) if -is_concealed(X).

    -accessible_to(X, Y) if concealed_from(X, Y).
    accessible_to(X, Y) if -concealed_from(X, Y).

    concealed_from(X, Y) if is_concealed(X).
    -concealed_from(X, Y) if -is_concealed(X).

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

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


    occurs(X) causes -is_concealed(O) if instance(X, expose),
                                         object(X, O),
                                         -defined_base(X).

    occurs(X) causes -concealed_from(O, B) if instance(X, expose),
                                              object(X, O),
                                              base(X, B).

    occurs(X) causes is_concealed(O) if instance(X, conceal),
                                        object(X, O),
                                        -defined_base(X).

    occurs(X) causes concealed_from(O, B) if instance(X, conceal),
                                             object(X, O),
                                             base(X, B).
                                            

    impossible occurs(X) if instance(X, expose),
                            object(X, O),
			    -defined_base(X),
                            -is_concealed(O).

    impossible occurs(X) if instance(X, expose),
                            object(X, O),
                            base(X, O),
                            -concealed_from(O, B).

    impossible occurs(X) if instance(X, conceal),
                            object(X, O),
		  	    -defined_base(X),
                            is_concealed(O).

    impossible occurs(X) if instance(X, conceal),
                            object(X, O),
                            base(X, O),
                            concealed_from(O, B).