Components
-----------

There are places and transitions, which are connected via directed arcs. 

Places can contain markers. 

Places and transitions can have names. 

Arcs may have numbers attached to them, showing how many markers are
used/generated by a transition. Arcs without numbers are assumed to
use/generate one marker.


Transitions may have expansions.


Generating Petri Nets
----------------------



Using Petri Nets
-----------------

Petri nets are useful for process modelling. 

If all input-places of a transition contain (enough) markers, the
transition can fire, which removes the markers from the input places and
puts markers into the output places.

Petri nets should be animated to show the operation of transitions


