theorem
  for K being Skew-Field holds id K is antiisomorphism iff K is Field
proof
  let K be Skew-Field;
  set J = id K;
A1: now
    assume
A2: K is Field;
A3: for x,y being Scalar of K holds J.(x*y) = J.y*J.x
    by A2,GROUP_1:def 12;
    J.(1_K) = 1_K;
    hence J is antiisomorphism by A3,Th22;
  end;
  now
    assume
A4: J is antiisomorphism;
    for x,y being Scalar of K holds x*y = y*x
    proof
      let x,y be Scalar of K;
A5:   J.y = y;
      J.(x*y) = x*y & J.x = x;
      hence thesis by A4,A5,Th22;
    end;
    hence K is Field by GROUP_1:def 12;
  end;
  hence thesis by A1;
end;
