theorem Th34: :: PROB_4:34

for Omega being non empty set
for Sigma being SigmaField of Omega
for P being Probability of Sigma
for D being non empty Subset-Family of Omega st ( for A being set holds
( A in D iff ex B being set st
( B in Sigma & ex C being thin of P st A = B \/ C ) ) ) holds
D is SigmaField of Omega