From: Ordered statistics-based list decoding techniques for linear binary block codes
OSD(I)
∑ l = 0 I K l
OSD(I1,I2)
∑ l = 0 I 1 K 1 l + ∑ l = 0 I 2 K 2 l
POSD(I)≡ISD(I)
POSD(I1,I2)