The inhibitor arc is an input with one logic annotation for the computation of the input’s status (annotation) (Hack, 1976). Unlike the “standard” input arc, it does not have a logic annotation for the computation of the mark (annotation) of the input place. The introduction of an inhibitor arc to a model changes the logic annotation for computing the status of the corresponding transition only; it does not add to the computation of the mark of the corresponding place.

If the fundamental equation is used to compute the marks of (input and output) places when transitions fire then the inhibitor arc does not affect the fundamental equation in the computation of marks; there are no changes to the fundamental equation of a model if an inhibitor arc is added to the model.

## Reference

Hack, M. Petri Net Languages. Laboratory for Computer Science (formerly Project MAC) Technical Report 159. [Electronic Version]. Massachusetts Institute of Technology, Cambridge, Massachusetts, March 1976. Retrieved on Oct. 5, 2013 at http://publications.csail.mit.edu/lcs/pubs/pdf/MIT-LCS-TR-159.pdf.