The classic Petri net — concurrency with feedback
Murata's canonical place/transition net — one transition forks a token into two concurrent branches that a second transition joins, with a feedback place closing the loop. The engine marks which transitions are enabled in the current marking.
For the systems modeller
This is the net you find on the first page of every Petri-net text (Murata's 1989 survey, Wikipedia's figure 1): the smallest diagram that shows concurrency, synchronisation, and feedback at once.
One fork, one join. Firing T1 puts a token into both P2 and P3 — two branches now run concurrently. T2 cannot fire until both branches deposit their tokens, so it is a synchronisation point (an AND-join).
The feedback place closes the loop. P4 -> T1 routes the result back as a back-edge, drawn around the layout so the cycle reads cleanly. In the current marking only T1 is enabled (highlighted) — T2 is dead because P2 is empty until T1 fires.
The render is downstream of the semantics. Schematex isn't drawing shapes you positioned; it validated the bipartite structure, checked each transition's input places against the marking, and coloured the enabled one.