PNS
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Is a public domain Petri net simulation tool
for Unix systems.
PNS is a graphics tool and requires the X window system.

Examples:
---------
- simple.net
  3-2 Reduction

- add.net
  x = x + y

- sub1.net
  x >= y : x = x - y
  x < y  : y = y - x

- sub2.net
  x = x - y
  NEGATIVE = 1   <=>   x-y < 0

- mult.net
  z = x * y

- phil.net
  Dining Philosophers Problem