Forwards Incidence
T0 T1
P0 0 0
P1 0 0
P2 1 0
P3 0 1
Backwards Incidence
T0 T1
P0 1 1
P1 0 1
P2 0 1
P3 1 0
Incidence
 T0   T1 
P0  -1   -1 
P1  0   -1 
P2  1   -1 
P3  -1   1 
Initial Marking
P0 P1 P2 P3
0 0 0 0
Current Marking
P0 P1 P2 P3
1 0 3 0

Enabled Transitions:    

Un-Enabled Transitions:     T0,   T1,