theorem
  (I --> f) opp = I --> f opp
proof
  set F = I --> f, F9 = I --> f opp;
  now
    let x;
    assume
A1: x in I;
    then F/.x = f & F9/.x = f opp by Th2;
    hence F opp/.x = F9/.x by A1,Def3;
  end;
  hence thesis by Th1;
end;
