module activating_and_deactivating
  depends on accessibility

  sort declarations
    activate :: make_accessible 
    deactivate :: make_inaccessible

  function declarations
    fluents 
      basic
        is_activated : entity -> booleans

  axioms
    is_accessible(X) if is_activated(X).

    -is_accessible(X) if -is_activated(X).


    occurs(X) causes is_activated(O) if instance(X, activate),
                                        object(X, O).

    occurs(X) causes -is_activated(O) if instance(X, deactivate),
                                         object(X, O).

    impossible occurs(X) if instance(X, activate),
                            object(X, O),
                            is_activated(O).

    impossible occurs(X) if instance(X, deactivate),
                            object(X, O),
                            -is_activated(O).