theorem Th38:
  for E being finite non empty set, A,B being Event of E st A
  misses B holds prob(A, B) = 0
proof
  let E be finite non empty set, A,B be Event of E;
  assume A misses B;
  then prob(A, B) = 0 / prob(B) by Th16
    .= 0 * (prob(B))";
  hence thesis;
end;
