reserve r for Real;

theorem Th28:
  for x,y being Element of 2 holds Set_to_zero.(x,y) = Set_to_zero.(y,x)
proof
  let x,y be Element of 2;
  Set_to_zero.(x,y)=0 by Th27
    .=Set_to_zero.(y,x) by Th27;
  hence thesis;
end;
