reserve E for non empty set;
reserve a for Element of E;
reserve A, B for Subset of E;
reserve Y for set;
reserve p for FinSequence;
reserve e, e1, e2 for Singleton of E;

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;
