  Design by Contract (DbC) and runtime enforcement of program assertions
enables the construction of more robust software. It also enables the
assignment of blame in error reporting. Unfortunately, there is no support for
runtime contract enforcement and blame assignment for Aspect-Oriented
Programming (AOP). Extending DbC to also cover aspects brings forward a
plethora of issues related to the correct order of assertion validation. We
show that there is no generally correct execution sequence of object assertions
and aspect assertions. A further classification of aspects as agnostic,
obedient, or rebellious defines the order of assertion validation that needs to
be followed. We describe the application of this classification in a prototyped
DbC tool for AOP named Cona, where aspects are used for implementing contracts,
and contracts are used for enforcing assertions on aspects.
