Every CALL event corresponds to an atomic goal, the one printed by the "print" command at that event. This atom has the actual arguments in the input argument positions and distinct free variables in the output argument positions (including the return value for functions). We refer to this as the call atom of the event.
The same view can be taken of EXIT events, although in this case the outputs as well as the inputs will be bound. We refer to this as the exit atom of the event. The exit atom is always an instance of the call atom for the corresponding CALL event.
Using these concepts, it is possible to interpret the events at which control leaves a procedure as assertions about the semantics of the program. These assertions may be true or false, depending on whether or not the program's actual semantics are consistent with its intended semantics.
If one of these assertions is wrong, then we consider the event to represent incorrect behaviour of the program. If the user encounters an event for which the assertion is wrong, then they can request the declarative debugger to diagnose the incorrect behaviour by giving the ‘dd’ command to the procedural debugger at that event.