reserve B,C,D for Category;

theorem Th5:
  for a,b being Object of C opp
  holds Hom(a,b) = Hom(opp b,opp a)
 proof let a,b be Object of C opp;
  thus Hom(a,b)
      = Hom((opp a)opp,(opp b) opp)
     .= Hom(opp b,opp a) by Th4;
 end;
