theorem Th21:
  for E being finite non empty set, A,B being Event of E st A
  misses B holds prob(A \/ B) = prob(A) + prob(B)
proof
  let E be finite non empty set, A,B be Event of E;
  assume A misses B;
  then prob(A /\ B) = 0 by Th16;
  then prob(A \/ B) = prob(A) + prob(B) - 0 by Th20;
  hence thesis;
end;
