theorem Th22:
  [!f,x-h/2,x+h/2!] = (cD(f,h).x)/h
proof
  [!f,x-h/2,x+h/2!] = [!f,x+h/2,x-h/2!] by DIFF_1:29
    .= (cD(f,h).x)/h by DIFF_1:5;
  hence thesis;
end;
