theorem Th18: :: MEASURE3:18

for X being set
for S being SigmaField of X
for M being sigma_Measure of S
for D being non empty Subset-Family of X st ( for A being set holds
( A in D iff ex B being set st
( B in S & ex C being thin of M st A = B \/ C ) ) ) holds
D is SigmaField of X