  The deontic logic DUS is a Deontic Update Semantics for prescriptive
obligations based on the update semantics of Veltman. In DUS the definition of
logical validity of obligations is not based on static truth values but on
dynamic action transitions. In this paper prescriptive defeasible obligations
are formalized in update semantics and the diagnostic problem of defeasible
deontic logic is discussed. Assume a defeasible obligation `normally A ought to
be (done)' together withthe fact `A is not (done).' Is this an exception of the
normality claim, or is it a violation of the obligation? In this paper we
formalize the heuristic principle that it is a violation, unless there is a
more specific overriding obligation. The underlying motivation from legal
reasoning is that criminals should have as little opportunities as possible to
excuse themselves by claiming that their behavior was exceptional rather than
criminal.
