Terminology

[Michael Weber <mweber@informatik.hu-berlin.de>]

Dear all,

this posting is in order to discuss the terminology of the
interchange format.  The discussion will help to clarify the 
structure of the used terms and their relations.  I hope, it avoids
misunderstandings of Petri net terms related to the file format.

First of all, a Petri net file may contain several Petri nets.
Each Petri net consists of Petri net elements.  They are divided
into Petri net objects and Petri net labels.  The Petri net objects
define the net structure.  These are places, transitions, (nodes), 
and arcs.  Whereas the labels are further elements of a net, such 
as a name of an object, a marking, a guard, a declaration, an arc 
inscription, and so on.  

Each label is necessarily associated with a Petri net object or with
the net as a whole.  We dsitinguish between two kinds of labels.  
The kind are the labels with potentially unbounded values.  They 
depend on the net type or other labels (name, marking).  These 
labels are called open labels. The second kind are the labels which
define the meaning of their objects (type of an arc (read arc, 
inhibitor), classes of objects as T. Mailund proposed).  These labels
are called predefined labels.  Predefined labels influence the
graphical representation of the Petri net objects.

A Petri net element has graphical information.  The graphical 
information (at least the position) of the open labels is relative 
to the objects at which they are annotated.

Petri nets may be structured.  We distinguish two kinds of structuring.
The first kind organizes a Petri net in pages.  Thus, a Petri net may 
consists of pages. A page may be also substructured in pages.  Pages 
are associated with each other via reference places and reference 
transitions.  Such a reference node has a graphical representation and 
is a source or a target for an arc.  But, a reference node has no 
individual (open) labels.  A reference place (transition) refers to a 
place (transition) on another page.  A reference node may refer to 
another reference node.  But, cycles of such references are not 
allowed.  A net organized in pages may easily transformed into a flat 
net (without pages).

The second kind of structuring is addressed to be a feature of higher 
levels of the interchange format.  It organizes a net as a module.  A 
module has an interface.  A module can be instantiated.  Such an 
instance is included in another net.  Petri nets can combine instances 
as they like.  A net which includes an instance refers to the interface 
of the module.  Pages are a special case of instances (explicit 
instances).  The module concept may be used to refine a part of the net.
This part will be replaced by an instance of the module.



Petri Net File Related Terms
----------------------------

Petri net file            -- the file storing Petri nets or
                             a PN module

page                      -- a Petri net may be organized in pages
                             a page may contain further pages

Petri net                 -- contains the elements building the net
                             a Petri net is related to a PN type

Petri net element         -- is a PN object, a reference node, or a 
                             PN label

Petri net object          -- is a PN node or an arc

Petri net node            -- is a place or a transition

reference node (place, transition) 
                          -- is a graphical representative of a node
                             it may be a source or a target of an arc

graphical information     -- The file, a net, a page, and an element may
                             have graphical information

Petri net label           -- is related to a Petri net or to a PN object
                             it contains a value describing a semantical
                             aspect of the net

open label                -- has potentially unbounded values which depend
                             on the Petri net type or other open labels

predefined label          -- is an attribute of a PN object or a Petri net
                             it has a value from a predefined set.

Petri net type            -- defines the PN labels of nets of this type
                             it is a predefined label of the net

Petri net module          -- is used to refine parts of another net
                             it has an interface and a (internal) Petri net
                             it is instantiated in a net which includes a
                             module 

instance of a module      -- is a Petri net which is instantiated from a
                             PN module

interface                 -- the referable nodes of a PN module
                             there are export and import nodes


The part ``Petri net'' of the terms (apart from Petri nets themselves)
may be ommitted in the context of Petri net files.


Open Questions
--------------

Are there more than one label of a certain kind on an object or a net?  
What does it means?  


Kind regards

Michael Weber
-- 
Michael Weber             Tel. +49-30-2093-3075; Fax +49-30-2093-3067
Institut f. Informatik, HU Berlin, Unter den Linden 6, D-10099 Berlin
mailto:mweber@informatik.hu-berlin.de 
http://www2.informatik.hu-berlin.de/~mweber