theorem Th92: :: MEASUR11:93

for X1, X2 being non empty set
for S1 being SigmaField of X1
for S2 being SigmaField of X2
for M2 being sigma_Measure of S2
for E1, E2 being Element of sigma (measurable_rectangles (S1,S2)) st M2 is sigma_finite & E1 misses E2 holds
Y-vol ((E1 \/ E2),M2) = (Y-vol (E1,M2)) + (Y-vol (E2,M2))