theorem
  (z1 - z2)*' = z1*' - z2*'
proof
  thus (z1 - z2)*' = (z1 + -z2)*' .= z1*' + (-z2)*' by Th32
    .= z1*' + -(z2*') by Th33
    .= z1*' - z2*';
end;
