This is the Petri Net code for Chris Crawford's conversation model as generated from APO. It's described here: conversation Petri Net

.name "Chris Crawford's conversation model"
.type PN

.places
L1
T1
S1
L2
T2
S2
p12

.transitions
t6
t7
t8
t9
t10
t11

.flows
t6: {1*S1, 1*L2} -> {1*L1, 1*T2}
t7: {1*T1} -> {1*S1}
t8: {1*p12} -> {1*L2, 1*S1}
t9: {1*p12} -> {1*L1, 1*S2}
t10: {1*T2} -> {1*S2}
t11: {1*L1, 1*S2} -> {1*L2, 1*T1}

.initial_marking {1*p12}

#petrinet