module unobstructing_and_obstructing
  depends on accessibility

  sort declarations
    unobstruct :: make_accessible
    obstruct :: make_inaccessible

  function declarations
    fluents
      basic 
        is_obstructed : spatial_entity -> booleans

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

    false if instance(X, obstruct),
             object(X, O),
             -instance(X, spatial_entity). 

    false if instance(X, unobstruct),
             object(X, O),
             -instance(X, spatial_entity). 
  

    occurs(X) causes -is_obstructed(O) if instance(X, unobstruct),
                                          object(X, O).

    occurs(X) causes is_obstructed(O) if instance(X, obstruct),
                                         object(X, O).


    impossible occurs(X) if instance(X, unobstruct),
                            object(X, O),
                           -is_obstructed(O).

    impossible occurs(X) if instance(X, obstruct),
                            object(X, O),
                            is_obstructed(O).