reserve I for set,
  x,x1,x2,y,z for set,
  A for non empty set;
reserve C,D for Category;
reserve a,b,c,d for Object of C;
reserve f,g,h,i,j,k,p1,p2,q1,q2,i1,i2,j1,j2 for Morphism of C;

theorem
  x1 <> x2 implies for p1,p2 being Morphism of C opp holds opp ((x1,x2)
  -->(p1,p2)) = (x1,x2)-->(opp p1,opp p2)
proof
  assume
A1: x1 <> x2;
  let p1,p2 be Morphism of C opp;
  set F = (x1,x2)-->(p1,p2), F9 = (x1,x2)-->(opp p1,opp p2);
  now
    let x;
    assume
A2: x in {x1,x2};
    then x = x1 or x = x2 by TARSKI:def 2;
    then F/.x = p1 & F9/.x = opp p1 or F/.x = p2 & F9/.x = opp p2 by A1,Th3;
    hence opp F/.x = F9/.x by A2,Def4;
  end;
  hence thesis by Th1;
end;
