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