theorem
  [!(r(#)f1-f2),x0,x1!] = r* [!f1,x0,x1!] - [!f2,x0,x1!]
proof
  set g=r(#)f1;
  [!(r(#)f1-f2),x0,x1!] = [!g,x0,x1!] - [!f2,x0,x1!] by Th25
    .= r* [!f1,x0,x1!] - [!f2,x0,x1!] by DIFF_1:31;
  hence thesis;
end;
