@MISC{NET_1.MIZ,
  AUTHOR = {Korczy\'nski, Waldemar},
  TITLE = {Some Elementary Notions of the Theory of {P}etri Nets},
  DAY = {10},
  MONTH = {August},
  YEAR = {1990},
  ADDRESS1 = {Warsaw University\\Bia{\l}ystok},
  SUMMARY = {Some fundamental notions of the theory of Petri nets
are described in Mizar formalism. A Petri net is defined as a
triple of the form $\langle
 {\rm places},\,{\rm transitions},\,{\rm flow} \rangle$
with places and transitions
being disjoint sets and flow being a relation included in
 ${\rm places} \times {\rm transitions}$.}}

