0 -> S
S -> 0
I_1 -> 0
I_2 -> 0
R_1 -> 0
R_2 -> 0
I_1p -> 0
I_2p -> 0
R_p -> 0
S -> I_1
S -> I_2
R_2 -> I_1p
R_1 -> I_2p
I_1 -> R_1
I_2 -> R_2
I_1p -> R_p
I_2p -> R_p
R_1 -> S
R_2 -> S
R_p -> S