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