The central topic of this thesis is the theoretical investigation
of the *object-net* formalism. This formalism extends usual ones
by allowing tokens with inner activity. These active tokens are
modelled by Petri nets again, so we consider "*nets within
nets*". The recursive nesting is unbounded, since the net-tokens
can again carry nets as tokens.

The possibility, to consider Petri nets as tokens has fundamental
consequences for the general theory, since the concept of a *Petri
net* is no longer an object at the meta-level - instead it
becomes an object of the formalism itself at theory-level. So, the
concept of "*nets within nets*" makes the theory of Petri nets
reflexive, i.e. it can treat itself as an object of interest in some
way.

The idea of using Petri nets as tokens is due to Rüdiger Valk
who defined it in the context of "*Task/Flow Systems*" where the
tasks are modelled as Petri nets being processed in the system which
is modelled also as a Petri net. Valk extended this approach by
dropping the restriction of task-nets being causal nets. The resulting
formalism is known as "*elementary object systems*". These
systems are called "*elementary*" since the nesting of nets is
still restricted towards two levels. This thesis studies the general
model with unbounded nesting structure.

The idea of using nets as tokens can be formalised in two different
ways. The first one - called *value semantics* - considers the
net-tokens as values marking a place, while the second one - called
*reference semantics* - considers net-tokens as references
towards the object net. The nomen clatura is borrowed from prgramming
languages where parameters can be passed using "*call by value*"
and "c*all by reference*".

First, the thesis investigates whether object-nets can be seen as a canonical extension of Petri nets, when nesting is not used. It is also analysed whether object-nets have extended power w.r.t. typical decidability questions like reachability or boundedness. The power gained from extending the model also depends on the semantics used.

Secondly, the existence of two different semantics is anlysed w.r.t. the inner relationships of both. The circumstances are studied under which one semantics can be simulated by the other one. Especially those processes are of interest that can be simulated in both ways, since the these processes only differ in the representation but not in the inner strcuture.

The whole analysis is embedded in the context of the foundation of
multi-agent systems. The theoretical results contribute to the design
of mobile agents, since object-nets can directly express fundamental
concepts like *concurrency*, *distribution* and
*locality*.

Object Nets: Definition and Properties (in german). Logos Verlag, Berlin. 2004.

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