
theorem Th6:
  for x1,y1,x2,y2 be Real holds [**x1,y1**] - [**x2,y2**] = [**x1 -
  x2,y1 - y2**]
proof
  let x1,y1,x2,y2 be Real;
  thus [**x1,y1**]-[**x2,y2**] = [**x1,y1**]+[**-x2,-y2**] by Th5
    .= [**x1-x2,y1-y2**];
end;
