for Omega being non empty set
for Sigma being SigmaField of Omega
for P being Probability of Sigma
for C being non empty Subset-Family of Omega st ( for A being set holds
( A in C iff ex A1, A2 being set st
( A1 in Sigma & A2 in Sigma & A1 c= A & A c= A2 & P . (A2 \ A1) = 0 ) ) ) holds
C = COM (Sigma,P)