Layered Modes

Modes were introduced in logic programming to diierentiate the input arguments of a predicate from its output arguments. This information can be used for verifying the most diverse properties of logic programs, notably absence of run-time errors and absence of deadlocks in presence of delay declarations. We introduce here layered modes, an extension of existing mode systems which allow us to enlarge the class of programs which can be veriied by using modes. In particular, we show that this extension allows us to better handle programs that employ a dynamic selection rule and programs that use incomplete data structures such as diierence-lists.