It is possible to extend an abstract aspect, but not a concrete one.
The concrete aspect PointTracing (Listing PointTracing.java) extends the abstract aspect
ExecutingObjectTracing (Listing ExecutingObjectTracing.java). However,
PointTracing cannot be further extended. This is actually a
minor limitation that can be explained as an enforcement of the
``abstract superclass rule'' [16], i.e., the rule
that only the leaves of an inheritance hierarchy can be concrete.
Every aspect hierarchy can be transformed into one in which all
super-aspects are abstract, although this is not typically
required by programming languages.