theorem
  bD(cD(f,h),h).x = cD(f,h).x-bD(f,h).(x-h/2)
proof
  bD(cD(f,h),h).x = cD(f,h).x-cD(f,h).(x-h) by DIFF_1:4
    .= cD(f,h).x-(f.((x-h)+h/2)-f.((x-h)-h/2)) by DIFF_1:5
    .= cD(f,h).x-(f.(x-h/2)-f.((x-h/2)-h))
    .= cD(f,h).x-bD(f,h).(x-h/2) by DIFF_1:4;
  hence thesis;
end;
