theorem Th36: :: PROB_2:36

for Omega being non empty set
for Sigma being SigmaField of Omega
for A, B being Event of Sigma
for P being Probability of Sigma st 0 < P . A & 0 < P . B holds
(P .|. B) . A = (((P .|. A) . B) * (P . A)) / (P . B)