theorem :: PNPROC_1:36

for P being set
for N being Petri_net of P
for e, e1, e2 being Element of N holds {<*e1*>,<*e2*>} concur {<*e*>} = {<*e1,e*>,<*e2,e*>,<*e,e1*>,<*e,e2*>}