  It is known for decades that computer-based systems cannot be understood
without a concept of modularization and decomposition. We suggest a universal,
expressive, intuitively attractive composition operator for Petri nets,
combined with a refinement concept and an algebraic representation of nets and
their composition. Case studies show exemplarily, how large systems can be
composed from tiny net snippets. In the future, more field studies are needed
to better understand the consequences of the proposed ideas in the real world.
