theorem
  (f1-f2)|X = f1|X - f2|X & (f1-f2)|X = f1|X - f2 &(f1-f2)|X = f1 - f2|X
proof
  thus (f1-f2)|X = (f1|X)+ (-f2)|X by Th51
    .= (f1|X) - (f2|X) by Th53;
  thus (f1-f2)|X = (f1|X)+ -f2 by Th51
    .= (f1|X) - f2;
  thus (f1-f2)|X = f1+ (-f2)|X by Th51
    .= f1 - (f2|X) by Th53;
end;
