theorem Th3:
  [!f,x-h,x!] = (bdif(f,h).1.x)/h
proof
  [!f,x-h,x!] = [!f,x,x-h!] by DIFF_1:29
    .= (bD(f,h).x)/h by DIFF_1:4
    .= (bD(bdif(f,h).0,h).x)/h by DIFF_1:def 7
    .= (bdif(f,h).(0 qua Nat+1).x)/h by DIFF_1:def 7;
  hence thesis;
end;
