Net hierarchies

It is noted that the existing approaches to net refinements have the disadvantage that the final schema net remains flat (like a program developed by step-by-step refinement) and the higher level nodes of the net disappear or all nodes of all levels remain on one level in the final schema. The author proposes a refining constraint and procedure which determines a net hierarchy as a real hierarchy of nets and allows the desired refinement of S-elements and T-elements. As a consequence, the conceptual schema can be viewed as a hierarchy of several modules, each module being a valid Petri net